I believe your best option is to restrict the Safari web browser under Settings/General/Restrictions (you'll need to setup or enter a Restrictions passcode). Disable Safari & Installing Apps (so they can't install a different browser).
I have an 8-year-old and have these restrictions (along with In-App Purchases) enabled on his iPad. If he wants to view stuff on the internet, he does it under mom or dad's supervision. We also pre-screen all YouTube videos as well...you wouldn't believe the language that's used by some of these children that post videos.
As Meg St._Clair mentioned above, you can also install a browser like McGruff so it's not too restrictive.