DxDP doesn't split them up on the receipt to show how many of child or adult meals you have, however it is still child meals and adult meals. The computer does keep track and you can ask at customer service or at most restaurants to verify the specifics.
YOu cannot use a child credit for an adult meal.
We usually do DxDP because we actually do eat that much (last trip we had 1 child credit left, that's it). However b/c we do a lot of signature dining a few things we do to help balance out - have fruit for BF (for DxDP we use snack credits for BF). If you're regular DP, then pay OoP for BF (unless doing a character meal).
For lunch we'll often have a buffet b/c then we're filed up and won't need anything other than a snack later. If we do want something else we buy 1 adult and 1 child CS meal (for 2 adults, 2 children, 1 baby). This saves 1 each child and adult meal credit to be used later. (Though this works for DxDP since it doesn't matter where you use the credits). For regular DP Eat CS with 1 meal per person to fill up one meal, but then choose a larger CS meal later (for instance Pecos has HUGE tacos that we can easily split the meals, or Flame Tree BBQ also has large meals, or personal pizzas etc) where you pay Oop - instead of having a TS meal that day. This will save TS credits by not using any that day, but also save CS to allow you to use them on a different day allowing you to still have 2 DP meals per day. Even if both are CS and one is smaller.
The points are pooled over the course of your trip. So for a family of 4 - with 2 adults and 2 children staying for 5 nights will have 1 TS, 1 CS, 1 Snack per adult/day and 1 TS, 1 CS, 1 Snack per child/day. for a total of 10 adult TS, CS, and snacks. and 10 Child CS, TS, Snacks.
A child can use an adult credit to get an adult meal (but hy do that?), but an adult can't use a child credit. However if you ordered child meals for an adult, they likely wouldn't say anything, but you also likely wouldn't be very satisfied. TS restaurants are a lot more likely to enforce the rules, whereas CS you can go up and order 4 kids meals, no adult meals - and as long as you have 4 Child credits, they won't say anything.
Something else to consider is to choose the most expensive restaurants to use for the TS and use the DDP for those. Then when you run out of credits, pay OoP for whatever's left.