The dining plan includes one table service meal, one counter service meal and one snack for each person for each night of your stay (not day). It is about $100 per day for two adults and two kids between 3 and 9. The table service meal either includes the buffet with drink, tax and tip (like Chef Mickey's and Breakfastosaurus) or one appetitizer, one entree, drink, dessert, tax and tip at order-off-the-menu places like Sci-Fi. Almost all restaurants are included. It can be a very good value if you are planning to eat a table service meal each day. For breakfast, Chef Mickey's is 18.99 for adults, plus tax and tip. For dinner, it is 27.99 for adults. With tax and tip, the dinner alone is almost the cost of the plan for the day, plus you get a counter service meal and a snack. We did dinner at the 1900 Park Faire in January - the cost for me, DH, DD7 and DD4 for one meal was $100 - that is an entire day on the Dining Plan.
Now for the bad news: If you do not want to eat table service every day or do not want to plan ADR's in advance, you might not get your money's worth. If you typically share entrees or wouldn't order an appetizer or dessert, it might be too much food and you can go for cheaper on your own. If one of your kids is 10 or older, they pay the adult price, even if they really just eat chicken nuggets (they would pay the adult price out of pocket as well, though, at least for the buffets). Some table service places count as two table service credits - Hoop De Doo Revue, Jiko, Cindy's table, some others I can't remember. Also if your table meals are mainly breakfast, you wouldn't get as much value as you would at dinner since dinner costs more. However, if you want to eat at Le Cellier (very popular on the dining plan), you could easily spend $50 per person just on dinner if you chose accordingly.
Check out allears.net to see menu prices of places you are likely to go, and determine how many table service meals you might want. The Dining Plan can be a very good value, and lots of people like to know that their food is paid for befor ethey get there. Allears has a good description of what is included. Good luck planning!