I'm sure one of the reasons the new iPads this year didn't get the fingerprint TouchID is simply it would be hard to make enough for all the iPhones and iPads they're going to sell. (And I've not looked into how thick the part is compared to the thickness of the devices, that could be an issue too.)
But another reason...
iPhones are very personal, you might let your kids borrow it, but they aren't really shared within a family, much less to a guest.
But iPads are much more often shared. One of the things that iOS has lacked, and hopefully will come in 8, is some sort of user accounts. So when the parents use it, they get Safari with their own bookmarks; their own email, Facebook, and other accounts; their own apps. When a kid uses it their apps are restricted to PG ones, Safari or email might not be there at all, and they can't install new apps or other restrictions. All settings the iPad can do, but right now it's globally, not per user.
Without something like that, TouchID that only unlocks the device is nice but a missed opportunity for instant and easy personalization. Kid's fingerprint--gets kids account. Mom or Dad--they get their own accounts.
All that together seems a worthy upgrade for iOS 8 and the 2014 iPads.
Even do a nice setup when making an account. A few choices that set the basics for what is restricted that you could customize. "Adult"=All "Teen"=R "Pre-Teen"=PG "Toddler"=G