So, I've been pondering this, because I find these things interesting and I'm a very visual person... It's sort of echoing everyone else, but I thought that maybe Minecraft could be a good way of visualising the concept. Building blocks on top of each other for addition, and digging holes for subtraction.

I even drew a very basic little diagram when I was having my breakfast this morning. :)

If she could get herself a METHOD where + means going up and - means going down, that should work? So the more minus, the further down she goes.