How about a Case Cheetah?
It locks, it has a swing guard, it's well made (in the USA I might add), and they make it in two sizes (the Cheetah and the smaller Cheetah Cub).
I would get a nice belt pouch with it as well. I find kids are less likely to loose a knife when they carry it in a belt pouch than if they carry it clipped to a pocket or loose in a pocket. As an added benefit, they seem to take well to the concept that, when they aren't using the knife, it stays in the pouch.
They start at around $70 for the standard versions, which I find to be pretty reasonable for quality knives from the USA.