I am a dentist in the GTA
There is no rule like this mandated by rcdso - you can be in the operatory with the child so long you have a mask. I know, my Chris Hadfield outfit is hideous, but it's the rules.
There's no limit to how many can be in the waiting room either, so long 2ft distance is maintained. I enforce the waiting room occupancy limit myself, politely ask them to wait in the car, in case an "outbreak" is declared at my clinic and I have to close, thanks to those morons with smartphones.