Child account with incorrect telephone number

I created an icloud account for my child a couple of years ago, just to limit use on her ipad, family sharing. About a month ago I started getting notifications to update the password on the email address linked on my phone. But the password didnt work, when I go to forget password a code is sent to a phone number I do not recognise - even though this account is linked with my account, I set it up, the last 2 digits of the telephone number are unrecognisable. My phone number has not changed, the same for years. And now I cant even delete this account - how do I get rid of this?