iCloud add own domain error with child mail address

Hi folks,

i wanted to add my own domain domain.tld to iCloud and ran into a problem.

Current Situation:

• I have a AppleID dad@domain.tld
• I created an AppleID for my daugther cuddlebug@domain.tld

Now when i try to add my domain domain.tld to iCloud and add my and my daughters mail address for confirmation i can confirm my own mail but not the mail from my daughter.

The error it says it's already assocaited with another account.

So, what are the steps to take now so that i can add my domain?

best regards

Rolf