Timer Lockout for iOS allowing multiple apps
There is a free and very simple way to limit time on iOS if you have an activation passcode.


Launch the Clock app
Set a Timer
Make sure that "Stop Playing" is set under "Sounds."


If a sound is set, naturally that plays. If "Stop Playing" is set, the device will go to the lock screen. Naturally, this will require your passcode or fingerprint!

This technique allows another user (my kids in this case) to use whatever apps they want, but only for a set amount of time until they need my passcode. You can also use guided access (linked above) to restrict them to only one app.