For us the DDE is a much better deal. We are saving hundreds over the DDP using the DDE card. However, this doesn't work for everyone.
Our eating habits are, breakfast in the room, CS lunch, TS Dinner with one appetizer to share among the family, everyone gets their own entree and nobody orders desserts (at TS or CS) we get snacks later on in the park that we usually share among a few (or all) of us.
Appetizers aren't included in the DDP anymore, but even with the inclusion of appetizers we didn't need one appetizer for each member in our party. We feel the same about desserts, one or two for a table would be more than enough and many times while choosing desserts nothing appealed to us, we just ordered because it was included and paid for.
When I looked at the menus, determined what we wanted to eat and then ran the numbers on my spreadsheet, it works better for us to use the DDE. With the DDP we're paying for food that we won't eat or don't want.
If we want to pre-pay then we'll get a prepaid credit card and use that for our room charging privileges.
Can I just buy one DDE, or do I have to purchase for all 4 of us?
You can buy one DDE card, so long as the cardholder is present, the discount will apply for the entire family. If your DH and two kids were going to eat without you one night, then he'd need an additional card in his name. Additional cards cost $25.