I agree with the previous replies. Remember that a blank CD only holds about 700 megabytes of data (maybe a little less).
I don't know how large your memory cards are. If you're trying to empty a 4 gigabyte (GB) memory card, that's 4,000 megabytes, which means you'll need 5.7 blank CDs.
If Steve's Girl is right about the $14 per CD, that's $84 in CDs that you'll be spending at Disney World to unload one 4GB memory card.
I haven't priced any memory cards lately, so I don't know how much memory cards are these days. But I know for sure that you can probably get quite a few 4GB memory cards at home for that $84.
Plus, it's not like you'll never ever use these extra memory cards. You can use them for your regular, everyday photography at home. And they shouldn't take up any extra space in your bag...just the size of a piece of chewing gum or a postage stamp!
Anyway, if you think you might need extra memory, I'd highly recommend that you get it before the trip.