Your e-mail is what identifies you as a human being here, and you will use it every time you sign in. Work, school and study addresses are taken away from you when you change job or graduate, and shared or role-based addresses (like info@ or team@) belong to an organisation rather than to you — the day you lose access to that inbox, you can no longer receive your sign-in code. You can change your e-mail later while you are signed in, but because we never store it in a readable format, we cannot recover the account for you if you lose access first. So pick the e-mail that will follow you through life.

If your child doesn't have their own e-mail, you can use the yourname+childsname@gmail.com trick. Most providers (Gmail, Outlook, iCloud, Fastmail and more) ignore everything between the + and the @, so the e-mail still lands in your own inbox — but it counts as a unique e-mail here. That way you can manage your child's account without creating a separate mailbox.