Hi K.,
I am a general dentist, so I can tell you the benefits and the downsides of sealants. They should be done on adult back teeth only (the molars and occasionally the premolars). Sealants are placed into the grooves of chewing surfaces of teeth in order to keep food/plaque from stickng to the grooves and feeding harmful bacteria which causes cavities. These are generally very beneficial, however, they can fall off or crack. The main reason for the sealant failure is that they have to be placed in a very dry environment, and you can imagine how hard it is to keep a 6 year old (or so) still and keep their mouth dry. If they fall off, they can easily be replaced. It is very very rare that cavities will form under cracked sealants. Your dentist will check you children's teeth every 6 months, he/she can detect cracked or lost sealants. The long and short of it, is that sealants are great, but the most important thing is good hygiene, cleannings and check ups and not too much sugar :)
Hope this helps,
Dr. I. Slipak