Well, MCPI is not open source, so you cannot know how it is structured. But my guess is this: There are 3 windows. One of them is the black bckground, and the other is an 'overlay' over the black one, displaying the graphics. The last is one that controls the mouse. The first 2 CAN be resized without problem, but I don't think that Mojang have incorporated the ability to resize the 3rd one. So, to scroll around the inventory, you have to use WASD and the enter/return keys. Black line? Many people have problems like this, and (no offence) it seems that only they have the problem. I myself have a problem with Android and XBMC. So I think that the black line is just a unique feature. I guess that you could try running the following command in the terminal (only run it is you have very little or no software that you have installed on the Pi after you got it):
sudo apt-get update && sudo apt-get upgrade
If that doesn't work, tell the kids that they have a very special Pi, one of the only that do exist.
Hope this helps. And check this MCPI map out; I made it myself:
https://www.dropbox.com/s/kxrrtmh39e7ie ... ky-Map.zip
Don't fall off the edge!
I'm happy to help.
https://www.raspberrypi.org/forums/viewtopic.php?f=78&t=51794 - List of games that work on the Pi.