Perhaps you don't want someone viewing videos on the YouTube App, or you don't think they should be browsing or trying to shop the App Store you can restrict them by using a numeric passcode.
All you have to do is navigate to the following:
Settings > General > Restrictions > Enable Restrictions
From there, you can enter a four-digit passcode. Make sure it's something you will remember, but not something others can easily guess.
From there, you simply toggle any Apps you want to restrict "ON."
Now don't forget your password.