You say that she "turned 3" since your last trip.
If you stayed on-site and she was on your last trip, she should have received a MB along with everyone else.
To answer directly, once a MB has been "assigned" to a guest, a guest cannot reassign
it to another guest.
The trick would be if your new AP MB would be given to you, "unassigned" (unlikely, but you could ask.)
(There is a procedure that Disney can use to reassign MBs, but it is not a common occurrence.)