If they're able to get a burner phone unsupervised then I think they could also pay an adult to do the face scan for them or borrow your ID from your purse to authenticate an account. What level of security would you need to totally prevent that kind of thing? Unless it checks your age every time you log in with biometrics I don't see it.
(Of course adding any level of friction will deter some kids, but needing to get a whole new device other than the one their parents gave them is already a lot of friction, isn't it?)