how to change my childs birthdate

Follow the steps here: Update the birth date associated with your Apple ID - Apple Support

If the birth date entered evaluates to less than 13 years of age you will not be able to change it.

Did you click on the link in the first line of Phil's post? That has instructions on how to change.

You are welcome.

You're welcome. Glad it worked out.

and thanks for the assist Eric.