How old are your kids?
Personally, I would not waste prime touring time eating breakfast at a bunch of character meals. I you moved some of those breakfasts to lunch or dinner how much do they cost? Pricing can be found here on the DIS.
I think it is probably close to a wash, but it depends on your whole eating plan and the ages of your kids.
Your four breakfasts would run about $400 plus tip.
You would get 6 sit downs and 6 counter service meals. Plus your mugs and your snacks.
For me the mug value is as a souvenir. I never drink soda and I don't drink Disney coffee.
Six snacks would cost about $20.
So now you have to eat an additional $450 worth of food to break even. 2 additional sit down dinners can easily run $125 to $150 each. so $250 to $300.
And, you have 6 counter service meals - lets say $15 per adult = $180 and 6 per child at $6 each - 12X6 = $72.
So,
$400 plus $20 plus $250 plus $180 plus $72 plus $10 value for the mug as a souvenir = $962 and you still have to pay the tip at the sit down place.
So for you it might work depending on whether you get so stuffed after the 3rd day you don't want to see a Disney menu again! (Which is what happens to me>>)
Tell us where else you have made a sit down reservation and it will be easier to get closer to the true total.
Now all this said.....I will only do a dining plan if I have a separate one night add on reservation (sometimes I book a night and have to move to another room)....then I'll grab dining plan and do a sit down somewhere.