If you want to make the decision based solely on money, take a look at the menus for places where you plan to eat and add up the cost of food at those places. Don't factor in tips or alcoholic beverages (you pay those separately with or without the dining plan).
DO add taxes as you will have to pay them, taxes are included in the dining plan fee.
DDP cost per night you stay at Disney: $123.96 for your family (3A, 1C).
On the plan you get a Snack, Table Service and Quick Service meal for everyone in your group. Say for example you chose this eating plan:
Late breakfast ADR at Chef Mickey's: $87.29 (includes taxes)
Snacks either before Chef Mickey's or after dinner: $12 plus tax = $12.78
Dinner at resort CS(chose Carribbean Beach Resort, lots of choices):
- Turkey Dinner $8.99
- Cheese Burger and fries (maybe for your 10yo) $5.89
- Fetticine Pasta with Chicken $7.99
- Pasta with meatball $3.99 ($5.19 for a more expensive kids meal)
- Drinks for everyone $5.56 (not sure, but let's say $1.39 each)
Total CS bill: $34.53 (includes taxes)
So for a rather inexpensive eating day you'd still spend: $134.60, more than the dining plan would have cost you.
This day would be considered "inexpensive" because:
- TS Breakfasts are the cheapest TS meal options
- CS resort dinners are often cheap
- Snack are just a breakfast item like a muffin, Mickey bar or popcorn for everyone, not too much to ask for a day in an amusemet park.
With the dining plan you could also share a breakfast platter, a lot of food and two beverages, then share a lunch combo meal. Then you could use your TS credit for really nice dinner which could easily cost $30-45pp without the dining plan.
One other benefit of the plan is that it is pre paid. It was so nice to know before my trip that our food was covered. Once on the trip it was nice to pick what I wanted and not have to consider price. With kids you won't have to tell them no or steer them too often so you will have to put a value on that.
One of the best memories is my nephew asked for an Itzakadoozie popsicle on our last morning at MGM. That is what he wanted for breakfast (11am). I said, sure we're on or last day of vacation. We used our last snack credit on that and had a real CS lunch meal an hour later back at the resort. He chose a second Itzakadoozie for his dessert there! This same child choose apple slices for dessert or snacks on previous days so we can live with the "treats" on the last day, no worries.
Hope this gives you an idea of prices and options at Disney! Good luck with your decision.