How to change a birth date for a child

Read this support article about how to update the birth date associated with an Apple ID: Update the birth date associated with your Apple ID - Apple Support

There's different ways to change the age on an account depending upon the age of the person, their target age, and the country's age definition of "child".

If the AppleID relates to a minor under the age of 13 years, you will not be able to change the date-of-birth yourself. You will need to contact Apple Support for assistance:

Official Apple Support