Interesting speculation, but fingerprint sensors are stupefyingly easy to fool.
I used to fool the first and second generation desktop sensors with gummi bears as part of our unusually far-fetched office pranks, and to this day every iteration I’ve come across has proven to be finicky, unreliable and, in general, a right hassle.
I do hope multiple accounts (or at least a guest mode) appear sometime soon. iOS is a pain to use in a family setting.