In Kentucky, a contract with a minor is voidable. In theory, upon reaching the age of majority, a minor can say, "I changed my mind, you need to pay me again." Although there is an exception in Kentucky law that says a bank may open an account for a minor and still have a valid contract, it does not carry over to the endorsement of a check.
Perhaps when you cash a check payable to a minor you are accepting a certain amount of risk. When you have the parent endorse the check after the minor you are essentially relying on the parent's warranties and obligation, not the minor's. Any risk is based on your inability to collect the funds from the adult. It's a reasonable practice, but it may be an excess of caution.
Custodians should never be endorsing checks for cash; their title says they are to hold the monies for the benefit of the minor and they have a responsiblity to account for the disposition of the funds.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.