Now that I've booked my parents on the 11/30 Pride is there any way I can purchase something like a prepaid credit card for them? Is there such a thing? Maybe something called Carnival Bucks, Cash or Gift Certificates? Or can spa services be booked/paid for in advance? They would never take cash.
Contact your TA and see if you can buy them a shipboard credit. But it's still kinda like just giving them cash, and they might feel insulted, unless they don't have the money to pay for onboard expenses.
Instead of doing that, maybe you could go to Carnival Guest Services and see if there are any Bon Voyage gifts they would like. I'm sure they'd appreciate those far more! A nice bottle of champagne (or three), a fine wine, some canapes when they arrive, that sort of thing.