I'm 23, and I think the Hello Kitty debit card is super cute and totally not childish. I'd love to get one, but my bank is Wamu.
However, I feel that the checks are a little too girly. I'd only have the debit card, because that will stay in my wallet and only I will see and use it. But you will write checks to lots of people, business, etc. So I wouldn't get Hello Kitty checks. But that's just my opinion.