Passcode on iPhone

If you forgot the passcode on your iPhone, or your iPhone is disabled or unavailable -

Click on the link for instructions on how to reset your passcode without a computer if you have iOS 15.2 or higher.

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

Click on the link for instructions on how to reset your passcode using a computer.

If you forgot your iPhone passcode - Apple Support

Going forward, you might want to restrict your child’s ability to change his passcode OR his AppleID.

See: Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

It’s specifically in the last section; “Allow changes to other settings and features”