My DS7 and DS4 get sealants put on their teeth 2x a year but the insurance company only pays for 1x a year. My youngest had to get 2 crowns and a cavity filled when he was 3 under sedation so we aren't taking any chances! I guess he inherited my bad teeth. My oldest is cavity free (*knock on wood*) The treatment only costs $18 per kid at my dentist. Great for my peace of mind.
I am a hygienist in Canada. Usually sealants are only done on 6 year molars because they have deep grooves and pits that plaque can get easily trapped in and children around 6 years old when they get them until about age 12 or sometimes even older can be horrible brushers. Kids are notorious for rushing their brush and missing lots of places such as the deep grooves of their molars. Sealants are just like plastic coatings that go in the deep grooves on the surface of the molar and prevent plaque from being trapped their for bacteria to turn into acid and decay the tooth. Worth every penny with kids.