That depends ... are your DDs princess-crazy yet? If they are, then I would say to try and book Cinderella's Royal Table for breakfast/lunch or the Princesses at Norway in Epcot. However, both those character meals are premium -- they cost more and have picture packages included. We've done the CRT lunch once when my DDs were 3 and 5 and that was enough for me! It was a guick hello and pic with each princess and that's that. If you must have the princess photos, its a good place to get them all at once.
My girls prefer Chef Mickey's and we go on every trip. It's become a Disney tradition for us. We book the first seating at a dinner time (we're not big on breakfasts), and we tend to do this meal on a non-park meal so that we're not rushed. The character interaction is great, and the atmosphere is party-like. The girls loved seeing the monorail go by, and we tend to ride it around after our meal. The first time we went my girls were the age of yours -- the DD3 was thrilled just to see Goofy. My DD1 was spoon-fed ice cream by Mickey.

We've always had great experiences there, and the girls consider it a highlight of their trip. Incidentally, these reviews are based on character interaction and atmosphere -- you're not really "paying" for the food