You can pull down on your home screen and search for it, or you can find it under Accessibility. Be sure to turn on Guided Access and the Accessibility Shortcut.
Find the app your child wants to use or watch, and triple-click the lock button, and this will bring up the guided access options.
If this is your first time using this tool, you will be asked to create a four digit passcode. Choose something you can remember, but that your child doesn't know ;)
In the bottom left corner, you will now see an options button. Enable and disable the features you want. If you want your child to stay on a video without clicking around, be sure to disable the touch!
Triple-click your lock button to turn Guided Access on or off. You can also change the options or exit the feature altogether.