Hi,
Thank you for writing to Microsoft Community Forums.
I understand that you are facing issues with Microsoft Family group.
Usually, when you add a Child account to Family group, the child account will not be able to remove themselves from the Family. Only the Adult account can remove the Child account from the family.
One of the possibilities that I can think of is that the Child account was added as an Adult account by mistake and that could be the reason your Child was able to remove him/her from the Family.
I would suggest you to try adding the Child account once again to the Family Group and see if the Child has an option to quit the Family group. Refer the article Add members to your family group
Let us know how it goes.
Regards,
Prakhar Khare
Microsoft Community – Moderator