We have to keep reminding DS8 that Disney is coming up so he doesn't spend the money he earns from household chores (feeding the cat, garbage detail, etc.) on other things. However, he often gets D$ for holiday gifts. Matter of fact, he got some on Easter from his Uncle. He usually gets some spending money from Grandma and Grandpa before vacations, too. At our spring garage sale, DS will sell his old toys for some extra money. It's amazing how many things he's getting rid of (thus making for a neater room) just to make a few bucks!
When we get to WDW, I will buy him his autograph book and a souvenier or two (I pick them from his list). On the first day, we encourage him to look around and think about what he wants. We remind him that we can come back to whatever store any time throughout our trip and that we have other parks to check out, too. Before we go, I also ask him to think about what he'd like to purchase (as he has ideas from other WDW trips). And, as another poster stated, they are quite savvy about how they spend their money once they realize it's coming out of their pockets and not yours. They really tend to "rethink" their purchases! And, we do try to "sway" DS not to purchase things that will not be used or things we could get at home (bouncy balls, DVD's of Disney movies, Mars Muck, Star Wars action figures). We always suggest things he'll get more use out of or play w/more or can't get at home (ie: Indiana Jones action figures). This past trip (or the one before?), he bought himself an Indiana Jones mug and a MM watch (among other things). I thought that was a great way to spend his $$. This trip, he wants to find a Stitch watch.
Also, DS has convinced himself that he does not like certain rides (ToT, HM), so sometimes we bribe him w/the chance to pick out a pin or get something small just to get him to try the ride. The last trip, I bribed him w/D$ to ride POTC. I swear, that child will do just about anything for the almighty dollar!
As another poster stated, we make a list of the item he wants, the price and where we found it. That way, DS can think on it and make wise decisions and see how/where he's spending his valuable money.
While the credit cards are great, you will need to help your child keep track of how much they're spending so they're not shocked to see that their money is gone. It's easy to just yank out that card and charge away! Esp. when you're young and you think the money is limitless!
Side bar -- all treats, drinks and snacks are picked up by Daxx and me. And, C.Ann's suggestion of the Outlet Mall is a super one! We've bought many items there at substantial discounts and it's so worth the trip!