Put the iPad into "single app mode"
iOS 6 introduced a new feature called Guided Access that allows the iPad to be locked into a "single app mode". Problem is, like most other things in iOS, this is buried in the Settings app.
You can find Guided Access in Settings > General > Accessibility > Guided Access.
Enable Guided Access and set a passcode, which can be different from the PIN code you use to access your iPad.
To enable Guided Access, fire up the app you want to lock into single app mode and then triple press the Home button. This will allow you to lock the iPad's user to this single app.
To exit Guided Access, triple press the Home button, and enter the PIN code you set earlier.
If you want to allow a child to browse the web using an iPad, then it is a good idea to make a browser available that keeps them focused.
A good app for the job is Sandbox Web Browser, which limits browsing to a predefined whitelist.
You can prevent a sneaky kid from switching browsers by either using the iOS restrictions to hide other browsers, or Guided Access to lock the iPad into a single app mode.
The Sandbox Web Browser app is free for a limited period; it previously retailed for $2.99.