Assuming that the minors are in fact independent contractors (which is always subject to potential legal action challenging such status), then your contracts would be subject to the state law in which the minor resides at the time that he or she enters into the contract.
In general, you would be subject to the possibility that the minor would later disaffirm the contract, at hs or her 18th birthday, at which point, whatever intellectual property rights may have been granted by the minor would be void, and you would be potentially infringing the minor's copyright with whatever you have done with the minor's creative work.
In California, however, it is legally impossible for a minor to act as an independent contractor where any intellectual property rights are transferred as a result of the contract. The minor is a statutory employee, and must be paid as such. Period, end of story. Family Code § 6750.
The point here is that in order to accomplish your goal, you will need an extremely thorough survey of the laws of each U.S. jurisdiction, so that you can avoid the myriad of potential bad outcomes. In my opinion, you would need to retain a large law firm with both intellectual property and minor entertainment labor law experience, and it would probably cost you $20,000+ in research and memorandum costs, just to find out where you stand on this issue -- even before you start drafting terms of service for your website.
BotXXXXX XXXXXne: this is a big project, so get ready to open up your wallet.
Hope this helps.
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