My husband and I charged everything to our room on our last two trips, which was then put on our cashback bonus card. We had, however, saved some money so we could pay it off the next month. It was extremely convenient and we only spent about 100 in cash money. This way we didn't have to worry about losing the cash. Even most of the faster food places took the room key.
Depending on how you plan on eating and how much souveniers you buy will depend on how much you'll spend on your whole trip. We averaged about 12 dollars for breakfast (very light at a food court), 20 for lunch (grab and go place), but 40-60 for dinner (Nice sitdown place which you could do for less without all the courses). We spent about 50 in souveniers both times. But you should always plan for extra if you are going to do the traveler's cheque route, since you can use them as real cash when you get back.
My recommendation is the room key credit card route though, keeping in mind not to put yourself in debt for the next few years with all the cute disney stuff.