I sadly have quite a bit of experience with this.
My first bit of advice is this: if the pediatric dentist won't allow parents to be with their children before, during,and after the procedure, FIND A NEW ONE. This is so, so important. They will have all these "great" reasons that they think its best for you, as the parent, to stay in the waiting room, but the bottom line is this: they don't want you back there because they don't want you watching how they (might have to) treat your child. This includes restraints (which were used on my autistic son, without my consent, and after I PROMISED him they wouldn't), screaming in pain at the injection of the IV (which sometimes needs multiple attempts on little ones), and screaming in pain at the injection of lidocaine (this is extremely painful for the filling of front teeth).
I got sucked into the "great reasons" by a renowned pediatric dentist in my area, and was assured that after my son drank the oral Versed, he would be knocked out. The rest would be a cinch. Well, I could hear him screaming for me during the procedure (which was blessedly short-15 minutes), but they wouldn't let me back there. Then I was told that only about 5% of kids don't get sleepy with Versed, and even though mine was one of them, there was virtually no chance he would actually "remember" everything after the drug wore off. Well, to this day, 4 years later, he remembers. He remembers that he screamed for me, and I didn't come. He remembers that they held him down, he remembers how much the injections into his gums hurt (2 front teeth fillings). He remembers.
We had problems with the same teeth about 8 months later. I went to 2 more pediatric dentists until I found one that would allow parents to comfort their child in whatever way they saw fit. The tooth needed pulled. I sat next to my son the ENTIRE time, and held his hand, and assured him it was almost over, and I was there. Yes, it still hurt to be numbed, but he had my hand in his hand, and it was a 3 minute procedure. He has very little memory of it, because he was calm, and there is no emotional trauma associated with it.
Please, please, find a dentist who allows you to be with your child if need be.
Also, beyond that, find a dentist who understands that besides a filling, if the tooth needs more work than that, just pull it. 2 of 4 dentists I saw for B's teeth said pull, the others said root canal. After much research, we decided pulling made the most sense in terms of pain, money and time. I'm happy to report he has 2 beautiful permanent front teeth now :)