How do I change the age on a family member itunes account ?

Hi, I just set up a 'family member' account for my son but inadvertently put the wrong date of birth, he's 10 but his profile says he's 6 and he's not impressed !, but I can't figure out to change it, tried logging in to apple with both his ID and mine but don't seem to be able to do it through either, can anyone help ??