Apple ID could no be created because ... is no longer available - Creating child's account

I am trying to add my child's existing Gmail address to an iPad as a child in my family on my AppleID account.

NOTE: Prior to this, my child created an apple ID account using her gmail address (the one I want to use as a child account) without permission.

When I found out, I decided to completely remove her apple ID account from that gmail address. It is the only gmail address she has, and I'd prefer to keep it like that instead of making an another address/account like an @appleid.com or whatever it may be.

So her AppleID account is now fully removed AFAIK, trying to login using her gmail address says there's no account, but when I try to add her to my iPad as a child in my Family I get the "Apple ID could no be created because ... is no longer available" message (... = her gmail address)

Is this message appearing because her appleID account has previously been removed?

Other points to consider: I am not an Apple user. I only made an appleID for my gmail specifically for this iPad device. It's the only apple device I have. The iPad was bought brand new.