Disconnect the spark plug wire. The gas-powered engine. They're small, and may not be seen by an adult, especially on a riding mower. Kids who are playing might run unexpectedly into the path of a mower. Kids are at risk for being struck and possibly injured by an object cut by a mower's high-speed blades.
Kids are naturally curious and like to watch or want to ride equipment.
Stop walking behind the mower. Wave to a friend. Remove both hands from the handle. Mow across the slope with a walk-behind mower. Mow up and down a slope with a walk-behind mower.