Your Nintendo Switch has parental controls, enabling restrictions on everything from online access to certain ratings of games. Or, you can be cruel and set daily play timers...and even cut your kid off after a certain time.
You can also do this with your phone, without even using the Switch. The Nintendo Switch Parental Controls app is available on Google Play or Apple's App Store. It can be used to set time limits on gameplay by day, and show what your kid is playing by user account. Setting it up requires a Nintendo account (which you already need for Switch) and an internet connection. From there, the phone app acts as the restriction gateway, not the Switch.
Daily game timers can be set easily, and bedtime reminders, too. The Switch can also be set to enter game-suspend mode at the end of the timer, requiring a password or parent override to unlock. Yeah, it's mean...but you could hop in and save their game if you're kind enough.
To remove these controls, just unlink the app. You're back to normal.
Watch the video above, and good luck.