When the plan first started that's how it worked. On our first trip we had 1 "child" and 1 10 year old who ate like a child. We used our credits all sorts of creative ways. For example, we payed OOP for him at 3 places and used the 3 credits to upgrade to a California Grill dinner.
Or we'd order 2 appetizers, 2 entrees, and 2 deserts and split them between the 2 adults and the 10 year old.
To give you a sense, my son (the child) and I went to Beaches and Cream. We ordered 1 drink (ice tea perhaps for me?) 1 appetizer (ceasar salad that was large enough to be a meal), 1 entree (double cheeseburger with tons of fries!) and 1 "dessert" (root beer float). They cut the cheeseburger for us and plated it as 2 meals (the equivalent of a whole cheeseburger, lots of salad, and lots of fries) and we used just one credit.
The one thing we did do was tip extra, so even though tip was included (I think this is a change too) we'd tip as though we ordered 4 meals, even if we only used 3 credits.
We asked several times whether what we were doing was "above board" and were reassured that Disney had designed it that way.
Sadly, that doesn't work any more, which is why we don't do the DDP any longer.