Okay this is somewhat complicated to explain. Long story short I'm divorced. My ex-wife had kids from previous marriage. Both her and the kids' dad reside at the same address. They've both encouraged me to stay involved and write to the kids which I have. Now my ex-wife doesn't want me to do so anymore but the kids' dad is fine with me doing so.......So I sent a letter specifically addressed to the kids' father and the kids and she told me she took it and threw it away......What do I do in this situation.
The only legal answer pertains to federal law about interfering with the mail. Most federal law pertains to taking of mail from mailmen, postal drop-boxes and the like. When it is delivered, it can go into a "letterbox", or a door slot. There is apparently no statutory definition of "letterbox", but it is interpreted or explained by the USPS here.
Except as excluded by 1.2, every letterbox or other receptacle intended or used for the receipt or delivery of mail on any city delivery route, rural delivery route, highway contract route, or other mail route is designated an authorized depository for mail within the meaning of 18 USC 1702, 1705, 1708, and 1725.
The crime Obstruction of Correspondence 18 USC 1702 says
Whoever takes any letter, postal card, or package out of any post office or any authorized depository for mail matter, or from any letter or mail carrier, or which has been in any post office or authorized depository, or in the custody of any letter or mail carrier, before it has been delivered to the person to whom it was directed, with design to obstruct the correspondence, or to pry into the business or secrets of another, or opens, secretes, embezzles, or destroys the same, shall be fined under this title or imprisoned not more than five years, or both.
If a letter is addressed to some other person and not you, then you are not the person to whom it was directed. This does not mean you have to sort the mail and take only your from the box, but you cannot take it with design to obstruct the correspondence. Assuming that the mail-disposer does not confess, you would have to establish (in court, not just persuade yourself) that the letter was received, and not by the addressee. Certified mail provides you with the signature of the person who receives is (forging the signature would be a crime as well).
This is not necessarily applicable to letters sent only to the children (assuming they are minors, and she has custody exclusive) – the clear crime would be obstructing correspondence with the father. Custodial parents have broad discretion with respect to their minor children, so she could well be entitled to filter her children's mail. The divorce agreement would be highly relevant in this case.