I have been lucky that my Aspie / won't brush properly child is also the one with a mouth that doesn't get cavaities easily. Still, after a boy died from undetected gum disease, I've been letting my son know that it is far more important that the obvious.
I think it's difficult for them to process the unmeasurable future benefits against the known immediate stress. My son will play games to get out of it, despite all he knows. I also think he would simply refuse to chew one of those tablets, telling me that "he knows it's there and he doesn't care." Hygiene is really something he doesn't enjoy and he doesn't "get."
I don't know if it will work with an Aspie, but my NT daughter has gotten really, really careful about taking care of her teeth after having to sit through 6 fillings. No fun at all. Still, the further away we get from those appointments, the less immediate the need starts to become, and she needs reminding. She actually does a good job, and lets us do the brushing at night. Despite all that, her mouth breeds cavaties ... it seems so unfair, since she has always been relatively good about it. The relationship between a good brushing job and healthy teeth isn't as clear as you might think: some kids are lucky, some are not. But, of course, it's better to cover the bases and do the best job of oral care you can. Which is, ultimately, one of those "maybe yes, maybe not" concepts that Aspie kids can't relate well to. No doubt, your kids are aware of some other child who never brushes and has no problems at all ... which makes the connection between action and result difficult for them to hold onto.
I would suggest making it a rule that you do the brushing, and then dispense with the tablets. Somedays a better job gets done than on other days, and that is the simple reality of it. Plus some days your boys will have greater need for you to go easy on them. If you take away the tangible measure, you get back that flexibility. But they will have to agree to the rule: allowing you to brush until you feel an adequate job has been done. If you really want them to practice the skill, get a timer and monitor brushing until the timer is over. If you deem they tried hard enough to do a good job, they get their bedtime story (for example). If you've been warning them through the process that they weren't doing it right, they lose the story (for example).
That basically worked for us, until I got tired of supervising / doing and decided to just concentrate on my daughter, since she has the cavaty prone mouth.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).