Remember - soccer is a game played by kids who can't use their hands
Seriously, any time the player initiates the contact with the ball and the ball hits them at the knee or below, it's a kick.
Sometimes you hear one of those genius coaches calling for a kick on the offense. Almost never would an offensive player intentionally kick the ball. I have seen it happen once or twice, just to prevent a bad pass from going OOB or something like that, but you're going to hear yells anytime the ball hits someone's leg or foot regardless of the situation. Just be prepared to enforce the rule as written.