Sharing phones seems like an absolutely terrible idea. It goes against the simplicity and good design that Apple pursue. I can think of tons of problems with this: being out of signal; having biometric data in cloud; running face matching across huge databases where false positives are extremely likely; interrupted signal where things are corrupt; designing efficient caching algorithms; sharing onboard storage with multiple users; security properties protecting that data...
Personal devices are personal for a reason. Simple, elegant, effective. Expensive, yes... but a better solution is to buy your kids cheaper phones.
Personal devices are personal for a reason. Simple, elegant, effective. Expensive, yes... but a better solution is to buy your kids cheaper phones.