Hi Oded!
I would strongly encourage a Dining Plan! Disney is a terrific experience, but they don't come cheap (smile). Any way you can save a little I say go for it, and even though you pay a larger up-front cost for one of the meal plans, you end up saving in the end. Disney offers 3 plans: Quick Service, Standard Dining, and Deluxe. You'll save 20% on Quick Service, 30% on Standard, and 40% on Deluxe, compared to if you paid out of pocket.
After vacations I skim over my receipts, the average cost at my table service meals for a family of 4 (2 adults, 1 child who's 6, and a son who's 3-so really it was a family of 3 eating wise) was about $100-$110 a pop, excluding tips.
On the plan, you'll save some money, plus you have the peace of mind of knowing you're already covered food-wise for your family.
Then all that discretionary funds can go to kid's toys (lol)!
Also, I always says reserve, reserve, reserve. If there are restaurants that are "must do's" you really should call and book a seat. If not, it's a risk getting a seat walk-in, and the more popular restaurants (like Chef Mickey's) is constantly booked!
Good luck on your planning. Have fun in September!