How do I change my child's account on family sharing to a child account?

I had set up Apple Accounts for both my minor sons using their own email addresses but my birthday. I know, big mistake. Now they just got watches for Christmas and I am trying to change their accounts from "adult" to "child" and it won't let me. Again, they have their own individual accounts utilizing their own email addresses, I just want to change the birthday so I can control the watches. I REALLY don't want to create new accounts for them as we have so much tied in to their existing ones. Help!

[Re-Titled By Moderator]