I agree...keep the reservation. When you check in, share your daughter's fear and ask if they can seat you in the main ballroom or the rose gallery...I would avoid the west wing. It is dark, and there is a picture that changes (when the lightning flashes) from the prince to the beast. It freaked my DS3.5 out. Our solution...since he specifically asked for the west wing was to sit him at the head of the table, so the picture was behind him.
The best only walks through once every 40 minutes or so, and down the center of the rooms, and does not approach/interact with guests. The remainder of the time, he is tucked in a small room to the side, where she won't even know that he is there.
It is a fun place, go for it. They make an announcement when he is walking through, so that will give you a moment to prepare her/take her to the bathroom if need be. It is over in a couple of minutes. Cool, but not a big deal.
We did lunch (FP) and dinner in December, and dinner was way better than lunch for us...and lunch was wonderful too.