You might try AoPS Beast Academy as it's more challenging than Khan for elementary math by focusing on deeper thinking and problem solving. My DD9 is pretty mathy (not speedy, so much as great reasoning ability), and was up to a 245 on spring MAP in grade 3, having mostly completed Beast Academy 3, and a third of Khan grade 4 (really because the computer isn't her first choice). I love the detailed explanations in the BA solutions, and I've even learned some new ways to think about math that I wish I'd known before.
