It only makes sense to get the DDP if it will be a value for your family. The only way to know that is to have an idea of what meals you will be eating and about what they will cost to see if you will save money on the DDP.
If you are planning to eat a quick-service lunch and a table-service dinner every day then there is a good chance the DDP will be a value for you. You also need to think about your eating styles. Do you generally order more expensive items on the menu, do you generally eat dessert with your meals, do you eat at a lot of buffets or mainly menu restaurants?
In order for the DDP to be a value for $39.99 per day, you need to spend at least $25 on a sit-down meal, $12 for a quick-service meal, and $3 for a snack. If your family are light eaters you may not spend this. If you eat at a couple of the more inexpensive breakfast buffets, you may not spend this.
The plan is definitely more of a value for kids ages 9 and under so if you have children that age it might work out better. If you have kids in the 10-12 age range you will have to pay the adult price for them so it may not be as good of a value.
So overall you need to try to estimate how much you will spend on your meals to determine if the DDP will be a value for you or not. For many families it is, but for some it is not.