if you want to use a shared email address, you could drop the unique validation for email addresses and allow a user to create multiple accounts providing that the user email and password is unique.
I would question a need for a child to login though. I might have a means for a user to register a student within their account and when they login you could allow them to switch between students.
otherwise, introduce a new field so you would have
- username
- password
then email can be non unique but combination of email and username would be.