Thought you were worried about who could use apps, so didn't say anything. But if you are trying to protect media, sure, it's possible.
GoodReader will let you protect folders. It's a recent update. Peled pointed out that it doesn't keep the protection when the files are copied back to the computer, but if all you want is to make sure they stay on the iPad. That should work.
Then, to make sure no one deletes GoodReader, go into the Restrictions settings and enable the prevent deletion options.
Then no one can delete apps, including GoodReader without the passcode to turn off Restrictions. And your files are safely protected by GoodReader itself. Since most file types can be viewed in GoodReader you can leave them there.
There are some other protected app out there that will password protect videos, photos, and such. Not familiar with them though, so you'll have to search for them yourself.