The founding fathers had no authority to bind anybody but themselves.
They didn't bind anyone. Each state was bound by its own ratifying convention deciding to ratify the constitution. As to whether these conventions had the power to bind others than themselves, well, they were chosen for that purpose by their state legislatures, and the state legislators were elected by the people of the state for the purpose of wielding the state's power to create laws and conclude treaties that bind the people of the state. As to whether people of the state who were eligible to vote had the power to bind those who were not, well, it's a tougher question, but centuries of democratic practice provide precedent for the answer being yes.
So the people and institutions that established the constitution had the power to bind others who were alive at the time of the constitution's establishment. What you're really trying to argue is that they had no power to bind anyone who was born subsequently. But again, that argument does not succeed, because, in fact, governments can and do assert power over people who were born after they were established.
And also it states in the Constitution “ No person except a natural born citizen, or a citizen of the United States, at the time of the adoption of this Constitution, shall be eligible to the office of President” The “at the time of the adoption of this Constitution” leads me to believe they did not want anybody become president that was not a citizen at the time it was adopted.
You misconstrue the purpose of this passage. The intention is to prevent foreign influence in the federal government. One way of doing that is to require presidents to be natural born citizens of the US. But in September 1787, when the constitution's text was finalized, the oldest natural born citizens were eleven years old and in no state to lead the federal government. Or maybe they considered people born before 1776 in the territory of the US to be natural born citizens and they just didn't want to exclude Hamilton and others like him (who had been born elsewhere but were established in the US by 1787) from politics. Therefore, they added a second criterion allowing those who were citizens as of the constitution's adoption to serve as president. Only one of these criteria need to be met, which is why the eighth word of the provision is "or."
To answer some of your questions in comments:
[If they had written] "we built these log cabins to provide safety for us and our [posterity]” would they have had the authority to bind their children to those cabins?
Yes, they can leave the cabins to their children.
The children would have had to live in them even after all their parents have died?
No, they can do whatever they want with them, just as the present people of the United States can amend the US constitution.
If you have no authority to tell another man what types of clothing to wear you can’t delegate authority to government giving them some imaginary right to have all men dress the same.
Nobody is claiming that the US has this authority. But it does have all kinds of authority that does not derive from individual authority. Rather, the people, collectively, have greater authority than any one of them does. That is a philosophical matter, however, not a legal one. Legally speaking, authority lies with the federal and state governments.
What is the constitution anyway...?
It is the document that establishes the government and creates the framework for its operation.
has the Supreme Court ruled it was a contract?
No. It isn't a contract, and individuals inside the US cannot avoid being subject to it (other than by moving abroad and renouncing US citizenship or by campaigning for and obtaining its repeal through the amendment process).
[Is] there a laws of contracts that defined it? As in the Death of the Signatories[?]
Parties to a contract can be individuals or corporations. The constitution is more closely akin to a treaty than to a contract, since it is an agreement between states, requiring ratification by the states' legislatures. But there is one sense in which it is similar to a contract between corporations: it is signed by individuals who represent the state, which is similar to a corporate contract being signed by one or more individuals representing a corporation.
If two people enter into a contract that ends because one of them dies, the significant fact is not that one of the signatories has died, but that one of the parties has died. For example, if Bob has power of attorney to sign a contract on Alice's behalf, Bob's subsequent death does not have an effect on the contract, but Alice's does.
Similarly, if two companies' presidents sign a contract, and one of them dies, or both of them do, the contract between the companies remains valid. Similarly, if two (or more) countries conclude a treaty and they both (or all) ratify it and it goes into effect, it retains its effect even after the people who signed it have died.
The same is true of the US constitution.