My co-founder handled the privacy policy so I've pinged him to provide a response. We'll get back to you as soon as possible!
EDIT: lynndotpy was right; it's required by law under COPPA if we have users under 13. It's the same reason that sites ask people for their age/birthday to check if they're under 13.
> We may collect the following information from you in order to provide our Services:
> ....
> Your child’s first name;
> Your child’s age or birthday;
Why?