Answer by John Burnett:
There is old guidance from 1995 (I believe) that said that a fiduciary is a "person on whose behalf ...," in addition to the person on whose behalf the funds are being held (the minor in this case). That guidance isn't available on the FinCEN site, and is therefore no longer official. However, it's still the best guidance I can offer you.
In the absence of official guidance, your preferred course of action is to contact FinCEN and ask. Make a note of whom you talk to and the response you receive.
Answer by Ken Golliher:
Fiduciary relationships confuse FinCEN. In an analogous situation, FinCEN has deemed that a deposit to an IOLTA or client trust fund account "benefits" the lawyer or law firm and directs they be listed on the CTR.
In the real world, a fiduciary does not benefit from a transaction on behalf of the party it represents. Here, apparently it does..