To set a volume limit for music and other media, in Settings, choose iPod > Volume Limit.
http://manuals.info.apple.com/en_US/iPad_iOS4_User_Guide.pdf
The only volume limit I'm aware of is the one for the iPod app:
That may work in this situation.
I have the same problem. Unfortunately the volume limit only works on music, not videos. It's a HUGE oversight. The odd thing is that I think it does work on videos on the iPhone.
Yes, I can't find a solution... I've asked an app writing friend to write an app for that ;-) It's a big oversight for parents of Ipad-smart kids !
2 year old turns up volume
Yes, someone please develop an app or something to lock the volume on the video on IPad. My daughter turns it all the way up. It would also be cool if I could lock a page so she can only chose from the icons that are on that page!