Use the Apple Watch as a Stop Watch
You can use the Apple Watch a stopwatch without the need to install any other apps or buy anything.
Apple includes a Stopwatch app that supports laps, which is perfect for running laps by yourself or if you are trying to track your kid running laps without going to a store to buy a stop watch for $5 to $15 that you have to remember to bring with you everywhere.
Press the crown to open the app screen on the Apple Watch, then tap on the small stop watch icon. You can start it with the button on the right and then you can even track laps. Swipe up to see a three lap indicator up top and to see digital times for laps.