If the fake child was set to an age less than 13 years old, you cannot remove it from the family Sharing group directly. You can transfer it to another Family group, or request deletion of the Child's Apple ID entirely through https://privacy.apple.com/
Log in with the fake child's account there (do not log in with yours), scroll to the bottom, and request deletion of the account there. An approval request will be sent to your Apple ID / devices. Once you approve the delete request, Apple can then delete the child account.
What Apple ID did it say was no longer available? Your husband's or your daughter's?
If your husband had your daughter's Apple IDs deleted then you will need to create new ones for them.
Check you are typing the email address / Apple ID correctly when trying to add them. If you are sure they are being typed correctly, then their Apple IDs may no longer exist. Which could be a problem if the Apple IDs are still signed in to an Apple device as they won't be able to be signed out and can render the devices unusable.
Apple IDs set to an age below 13 cannot exist outside of a Family Sharing group. If they were set to under 13, they either still exist in your husband's Family sharing group or he had them entirely deleted.
If they were deleted entirely, the add them the same way you added the fake child by creating new Apple IDs for them.