A:
A Bluetooth connection is a specific FM radio data signal that is encoded, though not encrypted. Each data signal is unique. Though the possibility exists for anyone to establish pairing with any device, once the devices are paired, there will be no interference from one Bluetooth enabled device to another.
In the case of your daughters, so long as each daughter pairs their own tablet with their own stereo, neither signal should ever interfere with the other. Stereo communication to each device should continue without interruption from any other device. Each tablet will only communicate with the device / stereo to which it is paired.