I think several parents in this thread might enjoy
Harry Potter and the Methods of Rationality if you aren't already reading it. This alternative-universe HP was adopted by an Oxford professor and his wife, and grew up with a prodigious love of science and rationality in general, which he then applies to his Hogwarts experience. He is homeschooled partly because he bit a maths teacher when he was in third year (2nd grade to you). "She didn't know what a logarithm was!" Lots of fun.
Some of our children might enjoy it too, but beware, some of it is not "suitable for children" - I remember a brief discussion of rape, for example, which I think I'll want to guide my DS through - and it's still being written, so who knows where it'll end up.
