My son (6) and I just returned from the dentist ten minutes ago. He was given the oral sedation (he also did this 3 weeks ago for the other side). The first time he did not fall asleep, was just relaxed. He did cry a bit, and I was there to hold his hand, but he doesn't really remember it ( they also put demerol in the sleepy juice, which is about 2 tsps of drink).
Today he wasn't asleep at first, but did snooze through half the time in the chair. He did wake up and cry a bit, but it went very well.
I was told this experience would be way more stressful for the parent than the child, and I have found this to be very true. Very stressful for me, but not really for him, and he's a very upsettable guy.
I don't know much about the laughing gas or iv sedation, but maybe there are other options out there if you are not comfortable with the ones you were given. Good Luck!