I think you have enough background on the track record of the account to believe that the $40 ATM withdrawal was done by the kid, and it's clearly on his own behalf, not on behalf of Dad or Mom. That makes the rest of the scenario moot, since the CTR trigger wasn't pulled even if you do attribute withdrawals to both joint owners.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8