I think the OP is right, for his family DDE may be better. They seem to eat the majority of their TS meals at lunch, thus making them less expensive. We do the opposite-all TS for dinner, and counter service for lunch. we also do a lot of TS breakfasts too. We were at the BWV Dec 1-9 and we kept really good track of our meals with the DDE discount just so I could compare it to the meal plan! For us, 2 adults & 3 kids, the dining plan wins by far. I think I would still get the DDE card to use for any table service breakfasts and alcohol, but our CS lunch, snack and TS dinner would be covered by the meal plan. We would absolutely save money. A counter service for lunch cost our family around $45 every day. Then you have everyone getting a bottle of soda and my daughters popcorn later in the day, around $15. Then dinner for 2 adults & 3 kids at places like Kona Cafe, San Angel Inn, Rose and Crown, etc. are around $120-$140. It's a big savings. Plus, we love the idea of meals being pre-paid and just going and eating!
I understand what some say about 10 and 11 year olds, but you'd have to pay adult price for them at buffets anyway, unless you lie about their age, which I wouldn't. Many people on the restaurant board have posted restaurants not letting 11 and 12 year olds order off the 9 and under kids menu or charging them double for a kids meal. In our case, for our Oct trip our oldest turns 10 about 3 weeks before the trip, thus making him an adult in the eyes of Disney. The meal plan is still worth it for us. He ate the adult Counter Service food every time anyway. The kids portions are really tiny at those restaurants. We'd have to pay an adult price for him at places like the Cape May Cafe Clam Bake or Ohana anyway, since he'll be 10.
I do like the DDE card too. We ate dinner twice at the POLY (Kona and Ohana) and sat at those little tables drinking tropical drinks waiting for our tables and the DDE card does take a nice little chunk off.

We also have a princess freak in the family and do both princess breakfasts. Although, when the price increase goes into effect for CRT, you can't use DDE anymore anyway. We also always do Cape May's and Crystal Palace's breakfasts and a couple Spoodles too. So, I think the DDE card may still be worth it in addition to the dining plan, as long as we have AP's. I'm just so happy about the dining plan though!!!