I haven't tried, but it seems there should be no App to do it due to copyright. Now, Google and YouTube no longer allows users to download youtube videos that way.
It should be easy to download the youtube videos using your Mac, then sync and transfer them to your iPad using iTunes. There are too many softwares that support downloading videos from Youtube on Mac. You might want to take a look on: http://kecodoc.com/best-app-software-download-convert-youtube-video-mac-os-x/.