family sharing

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".

In general, if you are using the term "child" as Apple does to mean somebody who is not allowed to manage their own account, you need to contact Apple about changing the age.