A marital agreement can be entered into both before marriage (a prenup) and after marriage (a postnup). And, it will have the same legal effect in either case once entered into by the parties.
In practice, marital agreements are usually proposed by a more affluent spouse and are reviewed by a less affluent spouse who will be giving up many of the legal economic rights usually associated with being married because the alternative is to not get married at all and the less affluent spouse thinks that marriage is still worth it even with fewer economic rights.
The governing principle of marital agreements is that they must be entered into with the highest level of voluntariness, good faith, and full disclosure.
Typically, a prenup with (1) limit the rights of the less affluent spouse in a property division upon divorce, (2) limit the less affluent spouse's inheritance rights, (3) limit the less affluent spouse's right to maintenance (a.k.a. alimony) upon divorce or legal separation, and (4) limit the less affluent spouse's rights to attorneys' fees in the event of a divorce.
Often, these limitations are phased in superficially more equal terms, but the agreement only has a practical affect limiting a spouse's legal economic rights in marriage on the less affluent spouse, when it is recognized that one spouse is starting out affluent and the other spouse is starting out less affluent.
The question states that: "I have kids so I want to protect them." So, determining precisely what you want to protect your kids from and how, in a manner that your spouse, courts evaluating the agreement in the future, and immigration officials, would consider to be legitimate, would be critical.
You should know what impact a death or divorce would have during a short lived marriage for someone in your shoes because you start trying to change the rules, because you might be surprised to learn that the status quo is less unfair than you believe it to be.
For example, in many states, property owned by one party before the marriage was commenced is not subject to division as marital property upon divorce and often the right to alimony following a short marriage that does not produce children is quite modest.
Also, if you are extremely worried about the possibility that your new marriage could fail and that this would force you to make economic sacrifices that you do not think would be acceptable under the status quo in the absence of a prenup, you might want to think twice about whether getting marriage in the way that you are planning to get marriage is really a good idea. Excessive worries about a break up that is damaging to you may be your subconscious's way of telling you that you have grave concerns about this marriage that could be deal breaking, as well as your outward positive hopes and dreams that may be unrealistic. You had a relationship or marriage that failed once. You may be ready for a more successful relationship now, but you need to be comfortable that the circumstances that caused your previous marriage to fail are unlikely to recur.
No prenup will ever make a divorce easy or painless, although it can bring some predictability to the process, if a prenup is well drafted and reasonable.
Typically, to prepare a valid marital agreement:
it must be in writing;
it will contain no invalid terms (e.g., in most cases, terms relating to child support or parental responsibilities, or terms that make marital fault relevant to the economic rights of the parties);
both parties will have reviewed a complete disclosure of the other party's financial circumstances;
each party will have had an opportunity to consult with a lawyer familiar this these agreements (at the other party's expense if the person agreeing to it can not afford a lawyer of their own);
each party will review the terms of the agreement in a language that they can understand, and
the circumstances under which the agreement is signed will not be under duress broadly defined (for example, to include presentation of the agreement on the eve of the wedding day without adequate time to consider its terms without undue negative consequences if the spouse does not agree).
Typically, the lawyer for each party would sign the agreement to confirm that all formal requirements and counsel regarding the agreement has taken place.
A marital agreement that doesn't meet the applicable legal standards (which vary state to state). I have cited the law in Colorado which requires parties to follow what would only be considered "best practices" in many other states, although there would be plenty of states where these legal standards would apply.
A marital agreement is still not valid if it is unconscionable to a spouse at the time it is to be enforced, for example, leaving one spouse unable to meet her basic needs and the other spouse wealthy after a long marriage.
There is no requirement that a prenup be signed in any particular country or place, so long as the person signing it understands that their signature is intended to be binding. There is no requirement that a prenup be signed by both spouses at the same time, as long as both spouses sign it before it is to become effective.
Realistically, as of 2017 in an average cost of living U.S. area, you are probably looking a legal expenses and related expenses involved in making a prenup happen given the need for Chinese speaking legal professionals and logistical issues, at an expense of $7,500 to $15,000 or more to get a prenup done under this circumstances. You would need to decide if this expense was worth the benefit to you.
It could be signed by your spouse while your spouse to be is in China, if the proper formalities are observed. In terms of the enforceability of the agreement, and the likelihood that it will be executed, this is probably the best option. But, as a practical matter this may be difficult to accomplish this because lawyers knowledgeable about U.S. law who can communicate in the spouse's topolect of Chinese may be hard to find, and Chinese attitudes towards prenups may be different from those of people in the U.S.
Negotiating a prenup in the face of a quite new relationship that often doesn't have a lot of depth at the start and is based upon mutual hope and a belief that the couple has shared interests as much as an actual long term relationship prior to getting engaged (which is common in marriages of the kind described in this question) can present a difficult challenge for the couple as they develop a marriage relationship very early on in their process when they don't have a lot of communication and relationship foundation to build upon.
Of course, there are exceptions to every rule. Suppose that you've known your girlfriend continuously since she was 14 years old and she is now 24 (you are 25) and both of you are fluently bilingual, because you grew up next to an embassy and she was the daughter of an embassy official and you went to prom together, and then the two of you went to college and graduate school together living together for five of those years. Suppose that she is from a wealthy family and needs the pre-nup to protect her family's wealth, and you are not very affluent despite being the U.S. spouse. Suppose that she's was a paralegal in a family law firm that draws up prenups when she was in college before moving back to China when she finished her studies, while you are a divorce lawyer, and that the two of you have talked about marital agreements for years. This might not be much of a relationship issue and might not pose the kinds of immigration questions raised below. But, if you fell into one of the exceptions to the general rule, you wouldn't be asking this question in the first place in all likelihood.
It is possible to obtain a financee visa which allows a foreign spouse to immigrate to the United States with an intent to marry upon arrival. But, in that situation there would be real doubt about whether a marital agreement signed upon arrival would be truly voluntary because failing to agree to the prenup and not getting marriage as a result would result in the financee's immediate deportation and terminate a wedding that the wife had already moved half way across the world and given up everything to participate in.
It is possible to execute a post-nup, but in that case, if it was truly voluntary, the wife probably wouldn't sign it because it would mean that she would be giving up rights that she had already acquired as a result of the marriage for nothing in return, and if it was involuntary, then it wouldn't be valid.
(There are circumstances when a post-nup can make sense for a spouse to voluntarily enter into after marriage, for example, to make rich relatives comfortable including a person in their Will or making a large gift to one of the spouses that is intended to stay in the family but will benefit the spouse during the marriage and the couple's children, but the facts in the question don't naturally suggest any of those circumstances.)
Unlike an ordinary marriage, which is presumed to be valid and in good faith, marriages which afford a spouse the right to immigrate to the U.S. and in due course, to become a U.S. citizen if certain other requirements are met, are carefully scrutinized for signs that it is a "sham marriage" and immigration judges can be quite skeptical in determining whether or not a marriage is a sham.
Couples go to great lengths to prove otherwise, and the burden is effectively placed upon them to show that they are not engaged in marriage fraud and instead actually married for love.
If a marriage with an immigrant is found to be a sham, the immigrant's visa will be revoked, the immigrant's citizenship application will be denied, and the immigrant may become deportable. There may also be sanctions for the citizen or permanent resident spouse who sponsored the immigrant.
One factor among many that can be considered when determining if a marriage is a sham marriage or not is the existence of a prenup or postnup, and even more importantly, its terms.
The more unfavorable the terms of the prenup or postnup are to the immigrant spouse, the more likely it is that an immigration official will conclude that a marriage is a sham for immigration purposes. There isn't a hard and fast rule, but it is one factor that is considered among others in reaching that determination. The key factors in evaluating a prenup for immigration purposes are the terms of the agreement. The more the immigrant spouse's legal rights in the marriage are limited, the more likely immigration officials are to view the marriage as a sham and the couple as engaged in marriage fraud.
On the other hand, if all a prenup does is to insure that a new spouse and children from a former relationship have equal inheritance rights and no rights upon divorce are modified at all, immigration officials may view the prenup as relatively innocuous.
There are a lot of good reasons not to get a prenup in the circumstances you identify if most of my hunches about the usual circumstances involved when someone asks a question of the kind that you have are true.
Before taking this step, you would be wise to consult with a lawyer to see what the likely economic and legal consequences of a divorce or death in the absence of a prenup would likely be in your case, as I suspect that your fears may be exaggerated or may involve issues that a prenup can't address. It could be that once you are well informed it is still clear to you that a prenup is a good decision.
But, given multiple indications that this may not be a good choice for you, I would strongly suggest thinking twice about this decision after conferring with a lawyer about your individualized circumstances before going forward with this plan.