Child’s age is incorrectly

On the child's Apple device: go to Settings > iCloud (Child's name and photo) > Name, Phone Numbers > Email, then click "Birthday". Set the birthday to the correct date, then click done. Hope this helps!

If necessary, contact Apple ID Support.

Apple ID Support

Apple ID Support (2) 

Apple Support by e-mail or chat    Work through the screens to get a chat option. 

Apple Support using Twitter

Or fill out and submit this form.          The form is for iTunes, but it will get passed to the ID Support section.