Let me stick my oar in (as parent of a mathy child and as a mathematician) and say that to people in the US school system Algebra may be the obvious place to go, but it is not actually where I'd be looking first, unless the child in question is especially interested in that area of maths, in which case of course follow his interests. I'd be far more inclined to go geometry, including both classic Euclidean proof stuff and more modern and way-out stuff like 4D polytopes, geometric approach to complex numbers, and off into topology. Also, logic, proof, probability... stuff that makes a change from things about numbers and near-numbers that they get stuffed with at school. Anne, if your DS hasn't already seen these fantastic Dimensions videos I thoroughly recommend them:
http://www.dimensions-math.org/Dim_E.htm
My DS also loves the Murderous Maths series, although it's possible they might already be a little young in style for your 8yo. (By which I mean, my 6yo finds their humour hilarious, but please don't tell me this is going to be his style for the next two years!)

FWIW my DS is not quite 6y2m and also has very little left to learn in 6th grade maths, I think (he'll finish ALEKS 5 tomorrow, and in looking ahead at the "new" topics in ALEKS 6, most are actually things he already knows, and he also knows a lot of maths that doesn't appear at this level). This is really still unusual in DYS? That shocks me, actually. Do you really have the impression that it's because something conceptual blocks children's progress, or is it just because most don't happen to go that way [e.g. you can't demonstrate that you can do calculations involving percentages if you've never met the word 'percentage', but would any 6yo DYS have any trouble after they'd been told what it meant? Is it GT denial to say that I doubt it?]


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