Guided Access allows you to lock into a particular app on an IPad. It stops children from changing apps if there is a particular one you would like them to focus on.
To turn on guided access go to settings - general - accessibility - guided access.
From there you can turn on guided access, select passcode settings and choose your own 4 digit password.
When guided access is turned on, open the app you want the child to use and press the home button 3 times, it will ask you to put your password in. Once that is done children can't leave that app until you press the home button 3 times and input your password again.