Hi, my son had to have it a few months ago, and he was fine. It was short term (about 20mins), he was semi-aware of what was going on, but it helped calm him enough to get him through the filling. He has a severe anxiety disorder (among other things) at 4 yo, so even a routine cleaning is rough.
DH said (I was working) that he jumped up and asked for a sticker within 5 mins. IME: Nitrous starts to wear off as soon as you take it off of the person you are using it on. I used it on patients before, and the change is almost immediate on adults, and a smidge longer on children. A very small percentage had some nausea while on it, or a headache, but overall it was safer for me to use on the little guys than the narcotics we had available to us. Good luck to your daughter!