Change birthdate in iCloud and attach to parent account
Hello. I set up my sons iCloud account on an iPhone and I used my birthday because it wouldn’t let me put in his. Now I’m trying to link that iCloud account to my family sharing and it’s not letting me because it has my birthdate on his account. How do i change the birthdate on his account so I can add his account to my family shearing?
iPhone 8 Plus, iOS 13