As col.bris suggests, you cannot 'password protect' regular iPad folders or (most) apps. So there would be no way of preventing your children viewing and/or modifying your calendar or contacts list. col.bris mentions the restrictions that you can apply in the 'Settings' app on the iPad 'Home' screen and that's about it.
You can prevent them from deleting apps too. If you 'Google' this problem, you might think there are some third-party apps out there that can do this but, if you study them in detail, you'll see that they're not really what you're looking for. These third-party apps allow you to protect designated data files - say documents you have authored - but they don't allow you to restrict access to the iPad's native apps or prevent access to the iPad's native folders, as far as I can see.
Maybe this will be addressed in a future iOS revision, since it's an OS issue. The iPad 'sandboxes' apps and this means they have very limited opportunity for interaction. The upside is that the iPad is effectively protected against viruses - the downside is that it's impossible (?) for a third party app to modify underlying OS functionality because to do so would open the door for viruses.
Tim
Scotland