The button is located on the chin. However, a shot to the jaw (or temple) from hooks can cause knockouts. Anytime the head rotates sharply due to the impact or the jaw takes a hard shot laterally, a knockout can occur.
As for getting kicked in the head, we are talking about a shinbone with significant wieght and velocity behind it (more than a punch), smacking a person in the skull. It's almost like getting hit with a baseball bat. The impact, due to the weight and hardness of the leg will cause a knockout.