DDP credits are not tied to an individual person, they are tied to a room. So, for example you do the regular plan (1 TS, 1 QS, & 1 snack per day) and 3 people are staying in a room for 5 days. That's 15 TS, 15QS and 15 snacks for the length of your stay. However, if one member of you party has a big lunch and skips one of their QS meals, then another member is free to use it instead. As another example, often I don't use several of my snacks by the end of the trip so I allow other people in my room to have them.
If you do the dining plan and your daughter leaves early, yes, other people can use the rest of her DDP credits.
We love the meal plan and we definitely save money doing it with our large family, but you might want to crunch the numbers and see if you'd save money by just paying OOP. I only say that because of your daughter leaving...it might be cheaper not to do it. If you okay with paying for extra credits and think you'll use them, I'd say go for it. It's nice to have everything paid for ahead of time and not had to worry about it!