If the child account wasn't properly set up in Family Center to begin with, there's a huge risk it might get banned regardless of any age verification.
The other thing to note, is that you may lose access to any games that were purchased on the non-compliant account, I don't think they can be transferred to another account.
To add yourself as the primary account, you'll need to factory reset the headset then set it up using your account first. Then use Family Center in the Meta Horizon mobile app, or on the Family Center website (click the big blue "Go to Supervision" button) to set up a parentally managed child account and add it to the headset.
Child accounts | Quest help (en-gb)
Create and manage a Meta account on Meta Horizon for your child | Quest help (en-gb)
Note: if the mobile app gives you a "something has gone wrong" error when setting up the child account, it's kind of normal. It takes a few days (~5-7) to verify your ID from your credit card details (it will charge $0.99 to your card and refund it). Also, after the child profile has been created and your ID validated, to add the child profile to the headset, choose the "send invite link by email" option. If your child is 10-12 yo, use your own Meta account email address. Then open the link in the email in your browser and follow the instructions.
