If your cats are as resistant to having their teeth brushed as mine are, giving them raw meat and raw, meaty bones is another way to keep their teeth clean. The abrasive action of the meat and bones rubbing against their teeth scrapes off some of the tarter and plaque.
My cats like raw beef. But raw chicken also works. Or give your cats raw chicken wings or necks. Never give them cooked bones. They can splinter and are a choking hazard.
While it would seem that "dental diet" dry food would accomplish the same thing, cats' teeth are sharp and pointy and are designed to tear raw meat (mice). Some cats swallow most kinds of dry food whole. Others splinter it with their teeth. Pieces of dental diet dry food usually get splintered rather than chewed. So the cats get no benefit from the food. But it does form a starchy residue in their mouths that encourages the growth of the bacteria that cause plaque.
Veterinarians say the best way to keep your cats' teeth clean is to brush them. But that's a nonstarter with my cats. If brushing doesn't work for you and your cats either, and if you don't want to give them raw meat, there are more suggestions for keeping your cats' teeth clean here.