How to change birthday for your child’s Apple ID account?

According to the support article: Update the birth date associated with your Apple ID - Apple Support

"If you need to change the birth date for someone below age 13 [age varies by country - see the article], contact Apple Support."

Click/tap on this link to contact support --> Contact - Official Apple Support

https://appleid.apple.com/

Sign into Apple ID with your daughter’s Apple ID and password. Select ‘Personal Information' and then select Birthday.

Why?