My mom was never allowed back with me as a child... and I had to have all but 2 baby teeth pulled because they wouldnt loosen up. At one point I was in so much pain that they had 4 people hold me down to extract a molar. This of course resulted in a horrid fear of dentists that caused me to not go go back for over 11 years.
Thankfully when I did go back to a dentist I only had one cavity and have a much better dentist now that I like.
If I had kids I would go back with them or they wouldnt work on my child. In my humble opinion a dentist that wont allow a parent with their child to keep them calm must be hiding something. Such as the terrible bedside manner that mine had and/or the lack of charachter to let a child in pain scream and cry rather than give them another shot.