As another poster mentioned...
www.allears.net has a lot of information. You can easily add up the cost but don't forget tax, and tip for table service. Lots of restaurant reviews there too.
If you're staying on a WDW property, I'd encourage you to consider one of the dining plans once you know how/where you want to eat.
You could spend less by eating more counterservice or by eating breakfast in your room or on the go with snacks. In 2008, we usually had breakfast in our room before heading to the parks -- we brought our own cereal and got milk from the resort, or got pancakes/mickey waffles from the resort - all of which is not expensive. Then we used the regular dining plan (Dining Plus) for dinner and some counterservice lunches.
For us in 2009, 2 adults and 1 child, our total food cost was an average of $200 per day. For 2 adults and 2 children, I'd estimate about $230 per day. But that is probably the most anyone would spend and is based on how we ate during our last trip -- all table service.
Here's how I came up with the average cost for our 2009 trip. Total Food/Dining Cost = $1820 for 9 days. We used the deluxe dining plan in Oct. 2009. The dining plan cost was $1320 for 2 adults and 1 child age 4 for 8 nights. It included 24 meal credits (appetizer, entree, dessert, drinks) per person and 16 snack credits per person, and 3 refillable resort mugs. Drinks did not include alcohol except at the dinner shows (hoop-de-doo, spirit of aloha, mickey's backyard BBQ). The dining plan includes tax but not tips. We ate all table service... the total of tips was $350. We also paid $150 for Spirit of Aloha instead of using the dining plan. We never paid additional for snacks or drinks because we used our snack credits.
Now that's a lot of eating -- but we dined at some places that charge 2 credits for 1 meal (signature restaurants and dinner shows). Most only charge 1 though.
With a 3 and 5 year old, I'll guess that you might want some table service meals with characters. In 2008, my son was 3 and enjoyed meeting characters in the park and at Crystal Palace.... In 2009, we made sure to have lots of character meals - all were fun.
In 2009, the deluxe dining plan was better for us (saved more money) because we did a lot more table services for character meals. And I liked that the deluxe plan includes appetizers... the plus dining does not.