Some online games only allow one game account per email (to prevent people from making lots of profiles & eating up server capacity, and to prevent cheating). But, I'm not sure how Fortnite/Epic Games works. If you're not sure what to do, you may want to email them. Tell them you already had an account, and now you're trying to make a second one for your kid with parental controls. Mention that the Epic Account ID's don't match when you log into EpicGames.