An energy transformation takes place any time that energy changes form. For example, carrying a cinder block up a flight of stairs increases the potential energy of the carrier and the block. Dropping the cinder block from the top of the stairs to the ground below converts the block's potential energy into kinetic energy.
The law of conservation of energy states that it is not possible to create or destroy energy within a closed system. When a child is running down the sidewalk but falls to skin his knee, the energy in the situation is not destroyed. Instead, it simply changes form. The child was burning calories, turning stored caloric energy into kinetic energy with each stride. However, falling to the ground stopped the consumption of caloric energy, and the kinetic energy at work in the child's motion went into thermal energy as the friction of the child coming to a sudden stop on the sidewalk creates heat. A similar response would happen if the child fell on grass, but the transformation would result more in thermal energy with the impact and less in terms of friction. This is why the child's injuries on a sidewalk would involve more damage to the skin.