It really does vary a whole lot. I've done trips for less than $25 a day for food and no souvieniers (didn't spend a dime on anything other than food, hotel, park tickets and a Mears shuttle), trips where we've spent $200 a day for a family of four (and two are preschoolers!) on food alone (as I've said before, last trip we spent more on umbrellas in our rum drinks than DisneySteve spent on food total). Trips where I've come home with $700 animation cells (and there are pricier souvieniers to be had at Disney), and trips where the souvienier is a tshirt off the back of the sale rack at MouseGears for $6.50.
Figure out how much you can afford to bring for spending money. Decide if your priority for that money is food or stuff (or both). And experiences - Cirque du Soliel will set you back a wad of cash, but if its what you really want to see, you can live off Cliff Bars and tap water for a week. Check the menus at the site listed above to get an idea of what food will cost you. If food looks like its going to cost more than your budget, read these boards for a lot of hints on how to keep your food budget under control.
(oh, I forgot, the priciest souvienier we ever bought was a DVC membership.....)
With your older kids, consider giving them their own souvienier and snack budget. That way, you won't be bugged for every snack, hat, tshirt that comes along. Your four year old may or may not be ready for his own "budget" - but may pick up on his brother's budgeting.