We are quite familiar with Beast Academy and AoPS. Beast Academy is actually AoPS for grades 3-5. From what you have posted in the past, I would suggest getting a book from Beast Academy for 3rd grade (where it starts) and trying it out. It has a comic book format that discusses the math (the text), plus a book of problems.
AoPS really starts with pre-algebra right now, so unless you feel that your DS is ready for that, I would wait a bit. Also, even the pre-algebra is QUITE challenging...probably much, much more so than most students are EVER likely to see in their B&M school.
