Descartes' Cove is meant as reinforcement/stretching with more challenging problems. There is a sort of book, "Descartes' Notebook" but I don't see anyone, let alone a young child, really teaching themselves all the material from it. It's a few pages of text, done up as an explorer's guidebook sort of thing. That said, you might be surprised how much can be learned without teaching, just from doing problems... My DS went through it slowly over nearly 2 years from his 6th birthday. Because he was learning in a lot of other ways at the same time, what happened was that in the early adventures he was meeting a lot of stuff for the first time, and often needed help from me, although by the later ones it was review (he still enjoyed it, though - some of the problems are significantly challenging, even if you do know the material it's based on). We liked DC a lot, but be aware that (a) the animation, though still impressive to my sheltered DS, look dated now and would probably not impress a child who plays video games routinely (b) there are a surprising number of mathematical issues. Most are just poor communication between mathematician and artist, e.g. I remember a q where the artist illustrated one definition of "diagonal" (of a polygon) but to get the right answer you had to use another definition. In the very last adventure there is a Hypatia's Cove q where the right answer is not available as a choice and the secret file solution is wrong, though! (Yes, I'm sure, no I haven't reported it, seems little point with something distributed on CD!)
For the OP, the thing I keep recommending like a broken record is
Alcumus, and I'll do it again. If you search, you'll find some of my other raves about it, so maybe I won't repeat myself. They have recently introduced a prealgebra section, btw, which lowers the entry bar and makes it available to younger children than it was, I assume.
