Explain the dangers of fire. It grows fast, the smoke can kill, and it cannot be controlled.
Teach them that fire is a tool for adults and matches and lighters are tools, NOT toys.
Create a home fire escape plan.
Model fire safe behaviors.
Keep matches and lighters out of reach (and out of sight).
In the garage, keep accelerants, such as spray paints, fuels, etc. locked up.
Supervise your children when they are around candles, campfires, fireplace fires, barbecues, and other open flames.
Take fire experimentation and fire setting seriously. If you do nothing, there is a high chance the behavior will continue and escalate. Unintended property damage, injury, or loss of life could be a result.