Cheryl, just had another thought, could you calmly take her next time and hand her over to the staff?
We always found little children were just a bit better behaved if mummy wasn't in the room at the dentists.
You could even say, "right, please ring me when emily's eyes are tested and I will come and pick her up." as you drop her off.
I know it sounds evil but it is for her own good and it isn't an invasive or horrid procedure.