Anyone know how to buy their kid a membership

I have a membership and my son now wants to go to some games. I've tried getting him a membership but if I do it under my profile it just says I can't because I'm too old. I tried setting up an account for him but it just says my email address is already used.

Obviously he doesn't have an email or his own payment details but am I supposed to invent one and set up a separate account or is there a more sensible way to add him onto mine.