I have 45 years in this industry. Yet, I continue to marvel at the nonsensical way banks deal with payments to children on a baseless assumption that a parent can act on their behalf.
Absent a specific state statute that says a parent can sign a child's name and create a binding contract (not likely), your bank is already assuming the risk that the payee did not get the money. Getting proof that the payee is a minor would not alter your risk in any way.
When Grandma writes a check to 18 month old "Buffy," I think Grandma is a bit touched in the head. However, I acknowledge that the funds could be deposited into an account in Buffy's name; e.g. a UTMA and no personal endorsement would be necessary. If Mom, Dad, or the homeless guy sleeping on a bench in the park wants to sign Buffy's name then his or her own name, then you have the promise of the last endorser that all signatures on the check are valid. If that turns out not to be true you could theoretically go after the last endorser. (Your efforts may be impaired by the fact you took the check with notice that the payee's signature was invalid.)
If Grandma's check is for $10 and you let Mom cash or deposit it into her personal account, you're taking $10 worth of risk. If it's for $10,000, that's the amount of your risk.
Does it make sense to make one decision if the amount is $X, but another if the amount is $XXXXX? If so, train your tellers well, making sure they know the exact dollar amount below which they use a fantasy as the basis for a business decision.