How to hide video on an iPad?
I feel that this is absolutely third-party app territory. One that fills your needs well would be OPlayer HD (not a Universal Binary, iPad specific), chiefly due to it's capability to password protect content, with a few very notable caveats:


Put the 'safe for everyone' videos in iTunes, sync it and use the Videos player for those.
Use OPlayer's sync mechanisms (iTunes Document Sync, WiFi Transfer, etc.) for getting the restricted videos into the app, and then password protect it. 


First, note that OPlayer does not support a simple pin/passcode, instead you have a full keyboard to type out a password. (You can always make your password a simple pin, I'm just stating that the input interface uses the full keyboard and not a "ten key" digit mode.)

Second, note that OPlayer's password protects the WHOLE app. Not just specific videos. This is why I suggest you split up the videos between the built-in player/iTunes housed videos, and OPlayer specifically.

Lastly, and perhaps most annoyingly, if you are done using OPlayer you should absolutely double-tap the home button and manually close OPlayer when you're done with it. If the app is running at all, even "suspended", you will NOT be re-prompted for the password when you return to the app.