A base runner may leave the base when the batter contacts the ball. Please, no 'sliding'. The base runner is out if struck by a fairly batted ball that has not passed an infield player excluding the pitcher area. Outs' are made by tags or by the fielder touching the base while in possession of the ball on a 'force' play. Do not throw the 'wiffleball' at the runner.
Base runners are responsible for avoiding all collisions with fielders involved in the play.
This includes whether the fielder is fielding, throwing, or catching the ball. Also, a fielder not involved in making a play is liable to be ejected from the game if they interfere with a runner or cause a collision. Infield fly rule.