How to buy membership for my daughter?

[Illusion35]

I’m a season ticket holder and want to buy a membership for my daughter. It won’t let me buy one logged in as me and allocating to her with the little pencil (she’s in my friends  and family) as it says her age is unknown? So I tried making her a whole new account in her name and entered her date of birth,  but it still gives me an error message. What am I doing wrong? 

[Saxondale]

I’d ring them up to be honest.