Most people know that they can set up a PASSCODE on their iPhone or iPad which has to be entered in order to use it. Just in case you have not tried it go to Settings, General, and slide down until you see Passcode Lock. This allows you to choose a 4 digit number that must be entered in order for anyone to use your iPhone. 
What many people may not realize is that there is a way to Restrict your iPhone so that only certain apps, and certain ratings of movies or TV shows can be used on the device. You can disable in app purchases and make it so that no one can download or delete an app unless they know to go to the restrictions list and have the passcode to change the settings. To go there choose Settings and next General. Under Passcode Lock you will find Restrictions. They can be turned ON or OFF. In order to turn them on you have to click the button at the top that says ENABLE RESTRICTIONS. It will walk you through setting up a passcode and then you can turn off anything that you do not want others doing with your iPhone.
When something is set to 'off' you do not even see it as an icon on your phone. For example, if the slidebar next to Siri says 'off' you cannot access Siri. There many items that you can control separately; YouTube, Camera, FaceTime, iTunes, Ping, Installing Apps, Deleting Apps, Siri, Explicit Language and many others
You can set it to stop using Location Services, or to only show certain apps. They are categorized by age, so you can set it up so that the only apps that show up are apps approved for 4 year olds and below. I am not sure who does the ratings and if all of them are correct, but it is better than nothing. If you are not sure which apps are good for different ages of kids you can take a look at Common Sense Media. They offer nonpartisan reviews of apps that are searchable.I recommend that you spend some time going through your phone settings to see all of the things that you can do. It is a great way to make the phone your own.
Here is a link to a slideshow that has additional ideas about setting up the restrictions on your iPhone or iPad.
http://www.slideshare.net/Papersh211/parental-control-on-apple-iphone
