The children may be familiar with times combustible material like charcoal and wood did not burn. We need to create the conditions under which they will burn, such as by squirting charcoal with flammable starter fluid, or using paper or fine kindling to start a wood fire. There are methods we use to reach the kindling temperature. Rocks and earthen materials are usually only charred unless the temperature is extreme. When substances containing carbon catch on fire, the carbon combines with oxygen to produce carbon dioxide.
Heat is released.
One calorie of energy raises one cubic centimeter of water by one degree Celsius. Light the candle and place it under the can of water. Place the thermometer in the water and carefully watch the temperature. You may need to stir the water a little. The wider the candle, the less length required to heat the water.