Fluoride is excellent at protecting a tooth from decay as it strengthens the surface of the tooth against acid attack.
The main way that fluoride gets to strengthen our teeth is from the toothpastes that most of us use. Regular use dramatically reduces tooth decay. Because very young children often can’t spit toothpaste out very well after brushing, they should use a special children’s toothpaste (which contains much less fluoride so it is safe to swallow).
Occasionally a dentist may apply a fluoride gel to a tooth surface which may be considered vulnerable to decay, or even try to reverse early decay that appears as white spots (next they become brown) on teeth.
Usually you can keep a child’s mouth free of decay by modifying their diet and improving their brushing.