Remove child photo from family share after deleting?

Did you remove the child from Family Sharing, as explained in this article?

How to leave or remove a member from a Family Sharing group - Apple Support

There might be age restrictions, not allowing to remove the account until it has been moved to another Family group.

Did you try to remove the account on the device by going into the settings for your Apple Account -> Family -> Tap on the "Name" -> Remove "Name" from Sharing Group?