How to change my child's birthday on family share

Hi There

See this instructions to charge the date of Birth of your child

To change a birth date to any age below 13,* you must be a member of a Family Sharing group, and your family organizer must approve the change. After you change a birth date to any age below 13, you can't edit it.* Learn more about Family Sharing. 

1. Update the birth date.
2. Follow the onscreen steps to send an email request to your family organizer.
3. Your family organizer will get an email with instructions to approve the request. 

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

I do not make the assumption that a "son" is necessarily a child, so follow the directions in the section that applies in your case.

Did you read the document to which I posted a link? I have never needed to change the age on an account but it seems you could be encountering a case where you must ask Apple to do this.