Is there away to add my legal child to my family share?

AFAIK, no. The child can only be associated with one family account and his father (in this case) would have to "remove" him from that account.

However, you may want to ask this question directly to Apple Support to see if there is another option available to you. You can also wait until someone else, who has come across the same situation, got it resolved and will chime in with what the solution was.