Our pedi dentist did not have parents in the back. I never had an issue with it as most of the people at this practice were ok with it as well. Quite personally, I like the set up because I don't want some parents I don't know and my daughter doesn't know hanging around where she is. I don't know if someone is a sicko or not. Plus is a parent starts freaking out about something, then that parent has scared all the kids in the room. Our Dr. was in an open setting.
Parents can be an intrusion to the staff, make the visit not go as smoothly and the experience not as good for the child. I know it is hard but we as parents, have to start letting go little bits at a time or when they move from the nest we'd go crazy! Basically, the dentists' try to set it up so that the children will have a good experience at the dentist and not have issues throughout their lives with fear of the dentist. A lot of parents (me included) prefer to do anything else but go to the dentist and I would never want my daughter to pick up on that and I don't think most parents would want that either.
Ours had a nice area that was for the children to color, draw, etc until it their turn, then when it came time for the dentist to talk with mom, the children were busy choosing their treat, getting new toothbrushes, etc.
As far as the gas issue, you should have been made aware of that beforehand. Why were they giving your child gas? Was it something other then a cleaning? If something other than a cleaning, I can see them using it but it still should only be used with parental consent. I know at my dentist, in the past, if he uses gas, I have to sign a consent form. As far as insurance coverage, I've never seen gas covered on dental insurance.
I would press to know why the gas was used but I would lighten up as far as insisting to be in the room.