This is really neat.
I was thinking of using something like
http://www.chorewars.com/ for afterschool so yeah, this is pretty similar. I might even implement a "monster battle" in the form of a quiz at the end of class, and if they get all the answers they earn their experience.
If you are worried about afterschool kids losing their books, how about keeping them in the classroom. Hand them out at the start and collect them at the end. You could ask if your school can pay for them, or just go to daiso/artbox/whatever your stationery store is called and get a bunch for under 5,000 won.
This also adds to the excitement of the first afterschool class, because you're giving them a present. And if you put their names on the books in big letters they will be easier for you to remember.