I keep "loaning" my finger to family members all the time, when I'm driving, watching TV, and they want to borrow my phone.
Not really sure Apple thought through this from every angle...
Oh boy, that's going to be awful fun for parents. Let the kid play with a game; they call your name while pointing the forward camera at you and you've just Face ID'ed some in-app purchase.
That's an interesting corner case. My phone has a setting that lets me turn off touch ID for App Store purchases (so I would have to use my passcode) so I guess you'd just have to use that if your kid kept tricking you and you couldn't stop them.