I need to change my daughters apple account to a child account

The only thing that determines a child from an adult is the age / birthdate set for the Apple ID.

And the only thing the age enables or disabled is the ability to turn on Ask to Buy.

If you must change the age of the account, follow the steps here from a device logged in to said account: link ➜ Update the birth date associated with your Apple ID - Apple Support

Note it does indeed affect some other things such as your ability to remove her from the family sharing group. If she is a minor she either has to be transferred to another group directly or you have to delete her account. She then loses the ability to redownload content.