Or...player B gets signed because he's new to the club and generates $3000 in dues. Player A is captive and they already have his dues, assuming he doesn't leave.
I'd imagine if Player A is already on the top team (playing EA on a U12 team) in an MLS Next club and doesn't get the MLS Next spot, he'll probably leave. Not always... but probably. So I don't think Player A is captive at all.
Because c'mon, parents are irrational.