You can set restrictions for some apps, and for purchased content. For example, parents can restrict explicit music from appearing in playlists, or disallow changes to certain settings. Use restrictions to prevent the use of certain apps, the installation of new apps, or changes to accounts or the volume limit.

Turn on restrictions. Go to Settings > General > Restrictions, then tap Enable Restrictions. You'll be asked to define a restrictions passcode that's necessary in order to change the settings you make. This can be different than the passcode for unlocking iPhone.