Having played it (and allowed my eight year old son to do so too), I'd have to say that the version you are offering free is too good. Why pay money for something you can get for free? (This is not a criticism of the game: it is fantastic.)
As a person who only plays games occasionally (so what am I doing on this site? I was curious) I can say that I wouldn't buy it because I can play as far as I can get (moon level at the moment) without having to pay. I have bought two games online -- Dweep and Snood -- because I was still interested when I got to the end of the evaluation section. Anyway, this is something for you to think about.
My son discovered that you can use the mouse, and I think it is much easier to use and allows getting to a much higher level. I think this should be made clear in the options. In fact, I think you might restrict it to just the keyboard to make it a bit more difficult.
I hope my comments are useful.