Like the previous poster, I would consider Agent Cool Blue - it's tinted rinse stains the teeth so they can see where they need to pick up the brushing to the next step. Since the dentist scare didn't work, I would go reality on them. Let them know that it's their teeth, and since they will likely need root canals, etc. from their decision to avoid brushing, you're going to have to stop doing allowance in order to begin a fund for the inevitable dental repairs. *Make sure you act bummed for them... like you don't want to do this, but you just can't afford to give them allowance anymore since you've got to save up. Let them know you might be able to help them out if they'll agree to forego the anesthesia. :)
I would google up a website showing photographs of a root canal, exposed nerves, etc. Maybe you'll find a video of the procedures. Let them see it. It may upset them a little, but considering how easy it is to prevent these problems...
Also, maybe run to Sams and buy a 12 pack of floss. We keep floss in the drawer of the coffee table, our nightstands, etc. We buy the Cleanpaste kind. It's tasty! Anyhow, it's not such a chore if you're catching your fave sitcom while you're flossing. I know it's not the most attractive thing to see people do, but our teeth are healthy and our teeth are flossed daily (usually several times).