Will your daughter be staying in the same room as you? If yes, then she has to be on the plan also. If she is not staying in the same room, then she does not have to be on the plan.
As was said, though, if she is on the plan, you can still use her credits even if she is not there. All credits are pooled together, and they don't care who uses what credit. As you get 2 meals per day on the plan, if you're a 3 meals per day family, you'll find use of your daughter's credits. You can even use credits to buy meals for strangers.
On our arrival day, once, as we were looking around the POR food court, someone came up to me and said, "you just get here?"
I said, "yes, we just checked in"
He said, "let me buy your family lunch to welcome you to Disney! We're on the dining plan, and checking out today, and have credits we aren't going to use."
He bought our 5 lunches, then when to find another family to spread some more pixie dust!
