DavidKingston wrote:
I found I was unable to set up a child account for her.
Why? What happens when you try?
If I just invite her then she will join as an adult.
No, when someone joins Apple Family Sharing they join as whatever age they have set for their Apple ID. They don't magically become an adult. If she's under 13, and has an Apple ID already, then she must be part of another family sharing group and you would need to request the other Family Organizer allow her to leave that group and join yours.
click here ➜ Move a child to another group using Family Sharing - Apple Support
If the Apple ID is already at 13, then she can just leave the current family sharing group and join yours.
click here ➜ Leave Family Sharing - Apple Support
Check the child's Apple ID's birthdate:
click here ➜ Update the birth date associated with your Apple ID - Apple Support
Apple says that you can control screen time and purchasing through Family Sharing up to the age of 18 but I see no way to do that.
Yes, once her Apple ID has joined your family sharing group you can turn on Screen Time and Ask To Buy
click here ➜ Set up parental controls with Family Sharing on iPhone - Apple Support
and here ➜ Approve what kids buy with Ask to Buy - Apple Support