The disney world web site lists all participating restaurants, etc. No difference in service. It costs $37.99 for an adult, and both of your kids count as adults. It includes tax and tips. It includes one "table service" meal, one "counter service" and one "snack item" per night of stay. You must purchase it for each person in your room.
A thought--if your kids would order kid meals at counter service restaurants, then you will may not save $ with the plan (since on the plan, they will pay adult rates). If your kids will order adult meals, you will save $.
If you eat at buffets, character meals, etc. you will of course pay for each person in your party at the adult rate. If you eat at places where you order off a menu, you can share a meal (whether you are on the plan or paying out of pocket.)
Whether the plan is a saving or not depends on how your family eats. A character meal in Dec. was $28 plus tax and tip. That make it about $33-34. OK, you only have to spend $5 for the combination of lunch and snack to make the plan a good deal. However, if you choose to go to Sci-Fi diner (instead of a character meal) and either share a meal with your child or order a kid meal for your child you could spend less than $30 for the meal and feed 2 people...in this case, the plan wouldn't save money.
If you use your table service credit for a character breakfast, the cost is about $22 per adult...again, not as good a use of the credits. It really becomes an individual thing based on how your family eats, especially in that gray zone of 10-12 year olds. Some eat everything in sight, others eat very little.
If you choose to not do the plan....there are lots of ways to save $ on munchies in the park. I put Rice Krispie bars in my park bag. ANy place that sells fountain drinks will give you a cup of ice free. You can then take it to the water fountain and fill it. You can buy a large size drink and have the kids split it (lots cheaper than 2 small drinks). Put a couple pieces of fruit in your bag. Seriously, before
DDP, I packed Rice Krispie bars or Kudos in my luggage because it saved me so much over buying Mickey bars!