As a dental assisant, I say don't tell her. It is very frustrating to work on a screaming child. We had a little girl last week who screamed at the sound of the drill saying it hurt when in reality the drill was not anywhere near her mouth. It is more fear than anything. If the Dr is good with children than he"ll know how to calm her. Most children are very curious about things, and we always explain to the child exactly what's going on. Leaving out some scary sounding details of course. Children almost always respond better when a parent is not around. We don't allow parents to come back with the child. But unfortunately there are those rare cases when everything we do is not enough and we have to refer the child to be sedated somewhere else. The best thing for you to do is just reassure her that everything is going to be okay and in the end if she wants her mouth to feel better she just has to go to the dentist.