Do you hand your smartphone to your child, then set this timer which force-locks when it runs out. So your child has to hand you back because phone and screen are lock.

This light utility app allows users to set a timer to lock phone and screen regardless of default system setting and whether user actively using or not. It will enforce entering password when the time is out.

At the start of the app the user has to enter the password; this is added to prevent "smart kids" to overwrite the the timer.

This app uses the Device Administrator permission. This permission is required to allow app to lock the screen when the time runs out.

App widget added allows screen-lock at the touch of finger. Try adding widget to your shortcuts.