It's not currently possible with the iPad, the only restrictions that are available to you are those in Settings > General > Restrictions (there is a bit more info about them here). You can try leaving feedback for Apple and maybe in a future update it'll be possible : http://www.apple.com/feedback/ipad.html
I haven't actually tested this possibility out, but I just did a search to see if there were any passworded 'secret folder' ipad apps like they have for your pictures and videos and one popped up. The description says for pictures and videos but they have a picture of the app and it appears to have an app inside of it. Anyway it's worth researching if you're interested. It's called Private Locked Folder by John Temple Group Pty Ltd. I know how hard it is to find certain apps some times so here's the link: https://itunes.apple.com/us/app/private-locked-folder/id498136041?mt=8 Even if you can't find one now, don't give up. Maybe they will take the hint that this would be a valuable app for many users. And if it happens that my suggestion sucked, please be gentle. lol.. Don't ream me too much for tossing out a suggestion that didn't pan out. I just thought it was worth looking into. Good luck!