Here is my experience, for what it’s worth, in dealing with a very fearful child and the dentist issue in particular:
My daughter (whom we later learned has Asperger’s Syndrome) was very fearful about a number of things as a child — like being in wide open spaces, crowds, church, gymnastics class — and the dentist. About ten years ago we realized, like you already have, that her response to these situations was not a discipline issue. We had tried to treat it as such because she would scream and throw fits, and was unable to verbalize her abject terror of these situations. Once we realized it was fear we were dealing with, we decided to do whatever we had to do to make her feel safe. We took her out of gymnastics and started carrying her or letting her ride in a stroller with a shade over her (she was almost 4 at the time and for some reason felt safer in our arms or hidden in a stroller) whenever we were in one of those scary situations. Within a few months we could tell that she was calmer and less fearful in general. I don’t remember how long we had to carry her around like that, but she did outgrow it!
For the dentist — I think we only took her once a year or so, and it wasn’t pretty. When she needed some work done, our dentist opted to send her to a specialist who could put her out completely, because they didn’t want her to get even more fearful than she already was. They had done the same thing with my oldest son a few years before, because he too would freak out at the dentist’s office.
So, for me the first step would be praying about how to help her feel safe at the dentist. Then brainstorming, based on what seems to help calm her in other situations. Role-playing like you mentioned may be helpful. Would sitting in your lap (with you in the dentist’s chair) help her be calmer? Also, if you can explain everything they’ll be doing at the visit, maybe go through the motions of counting teeth and cleaning them on a toy (even if it doesn’t have teeth!), that might help her feel less nervous. If the dental hygenist who works on her can explain everything in simple terms, let her hear the noises she’ll be hearing and see that it’s no big deal, that might help too. –Tara