It should have soft, polished bristles and be the right size for their hands and mouths, but otherwise, they might like one with their favorite cartoon character on it. Make sure to replace it when the bristles fray!
Sealants are recommended by the American Academy of Pediatric Dentistry, particularly for kids with a history of tooth decay already. They greatly reduce the risk of childhood tooth decay.
Whole or sliced fruit is a great mouth-healthy snack because the fibers help scrub the teeth clean. Cheese is a good source of calcium and stimulates saliva production. (We need saliva for neutralizing harmful acids and clearing away food debris.)
As much as we would all love having pearly-white teeth, make sure the teens know that whitening toothpaste only contains abrasives and polishing agents to remove surface stains, not deeper ones or white spots.