Accidently deleted my child from Family sharing, now i cant add her back. please help

Click on this link to learn how to 'Set up Family Sharing' --> Set up Family Sharing - Apple Support

Read the section: "Invite people to join your family"

"Remember, you can only join one family at a time, and you can only switch to a different family group once per year."

If the article does not help and you think she should be able to join, ask Apple:

Apple Support

if the child is under 13, you can’t remove them from Family Sharing. You can only transfer them to another family.

Leave Family Sharing - Apple Support