Wrong trusted number on child’s AppleID
My child’s Apple ID is locked and has an unknown “trusted number” associated with it. How can I access their account to add my iPhone number as the correct trusted number?
iPad Air 2 Wi-Fi
As is discussed in If your Apple ID is locked or disabled - Apple Support If you or someone else enters your password or other account information incorrectly too many times, learn how to regain access to your account.
You can reset the password on your child's device - which you have not specified. See If you have forgotten your Apple ID password. There are alternative methods discussed in the document.
Having reset the password you can also add a trusted number to the Apple ID account. See Two-factor authentication for Apple ID - Apple Support in which you will see:
Manage your trusted phone numbers
Update your trusted phone number on your Apple ID account page
- Go to appleid.apple.com.
- Sign in with your Apple ID.
- Go to the Security section and click Edit.
If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue. To remove a trusted phone number, click 
next to the phone number you want to remove.