It really, really depends....There are families of four that manage to spend $300 (or more) a day on food, and families of four that manage to spend $60 (and I'm pretty sure a few people here spend even less - by staying somewhere with a kitchen, packing lunches, eating dinner back at the condo, etc.)
Will you eat sit down a lot, or stick to counter service?
Do you snack (and will you buy them there or cart them in)?
Will you be drinking tap water (or bottled water you brought it) or pop you bought?
Do you think sit down meals need to come with appitizers, desserts and a glass of wine - or are you the type who goes to Don Pablos and get the appitizer quesadilla and drinks water?
Are you up for splitting meals? Disney portions (especially sit down) tend to be fairly generous (although not always, kind of eye the plates as you walk in).
Will you skip fries if they come with your burger?
Are you planning for three meals a day in the park - plus snacks - or one meal a day in the parks (breakfast in your room, big lunch, light snack/dinner in your room)?
When you go out for dinner, do you treat yourself to the most expensive thing on the menu, or do you always get the chicken or the pasta - because its only $12?
Are you going to do any special occation meals....dinner shows, character meals, nights out where you put the kids in a kids club?
Once you know these things, go to the menus at the link below.