Start by skipping the toothpaste. More important to just scrub them. Besides I find most people put way too much toothpaste on, I barely put a taste on or it causes too much foaming (we use Tom's of Maine for kids or just plain baking soda) Have you tried an electric toothbrush? My almost 4yo DS with SID has liked them better at times and at other times said they were too tickly. But it is worth having one around. His OT has a rubber toothbrush like tool that just has rubber nubs instead of bristles. It is not for actual brushing but it is used to help orally sensitive kids get used to something moving around in their mouth. She gave me one to use at home. We "look for monkeys". He opens his mouth and I use the tool to press down on his molars, pull his cheeks to the side, etc. My son does well now with the toothbrushing but we still play dentist sometimes where he lays back and lets me check and brush each tooth...we make a big game out of it.