If they're needing a dental done soon then it's proabably tartar on those teeth and no amount of brushing is going to get that off. It has to be scraped and chipped off.

However once they've had a dental done you can keep the tartar at bay a bit longer by 1-3 times a week brushing the teeth. You don't have to be as thourough as we do our own teeth, just the outside edges in the back near the gum line is fine.