Accessing child account

It’s been a long time since my child accessed his account, and it was logged out. I can still see it under my parent dashboard and change his password from there, but I don’t know the email to log onto his account on his device. Khan Academy doesn’t recognize his email for login, and when I login with my email, it just takes me into my own account. How can I figure out his login information or reset it (email also, not just password)?