Over on AllEarsNet, they have menus for many of the restaurants, including prices. I used those menus to estimate our food budget. My kids are 7 and 3.
Here's what I found out. When I estimated what I thought we'd actually spend (no appetizers/desserts, some days without table service meals) it came out to within about $20 of what the DDP would cost.
With DDP you would be entitled to more food (TS every day, dessert at CS, etc.), so I think it's a good value. However, we decided not to get DDP. We didn't *need* the extra food and I didn't want to schedule our whole trip around ADRs. If your family are "foodies", I think it's a good deal, but it didn't suit our touring style.
p.s. In reality, we ended up spending even less on food than I anticipated. Sometimes DD and I shared a lunch -- and the whole pizza from the hotel food court is a bargain! Fed the whole family for $15! So I was happy with our decision.