Adding parent account to child's iPhone after deleting previous account

So I decided to not use family sharing. Then got it again. I deleted my son’s account. Before I did that I forgot to put my account on his phone. Now I can’t do updates, allow him to download games.

How do I fix that? Or can someone from Apple access his new account and add it to his phone? I’ve never done this before and it’s frustrating. Please help

[Re-Titled by Moderator]

Original Title: Changed child’s account