Sorry you had such bad experiences. That really sucks.
Screaming kids grate on my nerves too. I think it's something that's hardwired by biology to insure that if a kid needs something, all he has to do is bellow, and any adult within earshot will jump to it so they can make the noise stop!

I've noticed recently that when I hear a kid screaming in the grocery store, my first reflexive thought is, "Thank God I don't have a small child!" Pity the poor parents! Actually, come to think of it, when my son was small I just got used to "kid noises."
I think that dentist needs to change his practice if he has kids shrieking like that all the time. My son went to a special pediatric dentist who didn't even allow parents to go back with the kids. I waited in the waiting room. My son started going there at 3 and I never ever heard any kid screaming there. Maybe the parents make the kids nervous if they're allowed in the room?
Sorry about the pissy staff member. If she doesn't like working with the public, she should get a data entry job somewhere in a back room.