If you need to convert a YouTube file to an iPad Video you can embed in Keynote, one of the best options is the app iCab Mobile, which is an alternative browser to Safari. By pressing and holding while a YouTube video is playing, you can download it, and the you can later save it to your photo roll.
Remember that videos for Keynote for the iPad have to come out of your Photo Roll. Yes, this makes no sense (shouldn't movies come from Movies?), and I've shared this with the head of iOS app development at Apple directly. Still…if you want to add a movie to Keynote, as of November 2012…it comes out of the Photo Roll.
Strictly speaking, taking video off of YouTube is illegal, but if you want to have a file for times when you do not have the Internet present (many schools), it is a logical solution (It also works well if a TV series is on YouTube and you want that series on your son's iPod Touch for a long car ride–just saying).
As a side note, I've been shocked to see how many full length movies appear on YouTube (in differing quality). I tripped on the full version of the original Toy Story at one point–there is no way that is legal.