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 it you back because the 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.

To start, cancel or modify are protected by the default password to prevent "smart kids" to overwrite it.

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.

UNINSTALL INSTRUCTIONS:
v3.7+
Tap "Uninstall the app" from the app's menu

Old version:
1-When you tap uninstall you will receive a "Uninstalling .. unsuccessful" notification from "package installer"
2-Tap on down arrow on the notification
3-Tap on "Manage device admin apps".
4-Un-check this app from "Device administrator" list
5-Tap on "deactivate" on the new page comes in 4-
6-Then go back to app icon, and uninstall the app as usual.