I highly recommend Beast Academy. It was specifically developed not long ago for math-advanced kids in grades 2-5. Students that are interested can then transition to AoPS, a long-standing advanced math site for middle and high school students. My D didn't find out about it until 7th grade, but it has been very beneficial.
(OT aside - frannieandejsmom, what's the standard math sequence at your school? My D had Algebra II in 8th but was just one year ahead of the normal honors track, and we chose not to jump her one more year with some of her peers. 4 years ahead means Algebra 2 is standard for HS Seniors? I'm just curious, as our mainline math sequence is Calc. senior year.)
