Minors (under the age of 18) cannot legally execute a contract (which is what that is - an agreement between the parties). Their parent / legal guardian must do so.

Indemnification / hold harmless agreements are not allowed under US Sailing prescriptions. By indemnifying a party, you agree to reimburse their expenses in the event of a suit for damages. For example, someone at the event gets drunk, falls off a dock and drowns. Their family sues the organizer. If you signed an indemnification agreement, the organizer can then come after you to help recover its costs - even if they win! (Lawyers ain't cheap.)

That's why you should never sign anything that has the words "indemnify" or "hold harmless". Your own liability insurance may not cover it.

As for the "when to race" component of your question, it comes down to the judgement of the OA and the PRO. The PRO runs races as directed by the OA (RRS 90.1). When kids are involved, it is much better to err on the side of caution.