There is currently no risk in the situation
@CMNJ described above. If it would make you feel more comfortable, you could transfer those unused FP to the family members that are still in the park rather than switching MBs.
Again, I am commenting only on the exact situation that was in question. If family members enter the park then decide to leave and want their family members that choose to remain in the park to use their FPs.
There are only 2 ways this would not be possible in the future.
1. Fingerprint scans at all FP attractions entries. (Which would slow down the FP lines to the same snails-pace as the front gate)
2. If Disney began to require guests to "scan out" when exiting the parks.
I suppose a third way would be if MBs were geo-tracked and the system could identify the guest left the park and is physically outside walls/fences of any given park. But in reality this would never work because many, many people have multiple MBs assigned to the same profile for various reasons. Some collect them, some have a special holiday MB they wear during the holiday season, some have multiple MBs from previous trips still attached to their profile.
So yes I believe what
@CMNJ described is perfectly ok to do.