A.V.
I would talk to them about the reasons they think that general sedation is required and get a 2nd opinion. When DD was 4 we chose the OR route for 7 fillings and a possible baby root canal. She has since had smaller fillings and a cap done without general sedation. They used nitrous on my DD but no demerol, etc. They did use lidocane or whatever the shots were (have to look) so she was not in pain. It sounds like what you are describing is not in an OR but in the dental office? When you say "sedation" I think of going to the OR and being put under general. Or do you mean that it's just routine office stuff? Is it the nitrous that could make her stop breathing? I would think they could do much of her routine fillings without major sedation at this point.
ETA: I am also wondering why this is a 3 part process. When DD had her 7 at once, that was to keep her from 4 sessions in the chair.