Whatever you find when you look at Missouri law, it's doubtful that there will be anything that says a parent can sign a child's name and, thus, bind the child to a contract. Only bankers believe that to be a possibility.
If it's a "joint" account, the kid needs to sign his own name and his access to the account would be the same as the other "owner."
The account where the adult retains control, but the money belongs to the child and the interest is properly reported to the child is the Uniform Transfers to Minors Act account (UTMA). (The statute directs that the money be delivered to the child at a specified age.)
Generally, a checking account is inappropriate for a UTMA.
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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.