Okay. Just to let you all know that I had the same issue. I created a normal account for my daughter when she was 5 years old and before I knew about the family account creation functionality. The other day I corrected the date of birth and boom I was locked out!
Only option I had was either submit a passport or creditcard scan as proof of age. I opted for passport since my daughter has my surname and her backup email is my own Gmail account which correlates to my passport details.
And I’m now happy to say the account is now unlocked! :-)
Do note that you won’t receive an email confirming it’s been unlocked. Just try logging into the disabled account after 24 hours!
Hope this helps. Good luck!!!