Disney card with money on it. I told them that it was enough for food and snacks and souvenirs. If they wanted extra, they would have to spend their own money. I also told them that any money left on their card at the end of the trip, I would give to them.
It was fascinating to see the difference in the way they approached spending. I'll never forget my 7 year old daughter running into one of the stores and seeing a stuffed animal she wanted. She looked at the price tag and after the sticker shock sat it back down.
Pretty amazing even though it wasn't their money.