How do I Change child’s age?

Hello. If the child already has a date of birth listed as below 13 then for privacy and security you’d have to contact Apple support for this. For more details, including how to contact Apple Support see this help page

Update the date of birth associated with your Apple ID – Apple Support

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name].
2. Tap Name, Phone Numbers, Email. You might need to enter your Apple ID password.
3. Tap Change Birthday or Birthday.
4. Update your birth date, then tap Done. If you're changing a birth date for a child, learn what to do.

Update the birth date associated with your Apple ID - Apple ...