Math skills build directly on top of each other more than most subjects — this hub groups every math resource on the site by where it fits in that progression, so it's easy to find exactly the right level.
A child who answers correctly but slowly and with visible effort usually needs more practice at the current level before adding a new layer of difficulty on top — see Homeschool Scope and Sequence by Grade for typical benchmarks by grade band.
Roughly: counting and number recognition, then addition and subtraction, then multiplication and division, then fractions, then pre-algebra concepts. Each stage depends on real fluency in the one before it.
Fluency, not just correctness — a child answering slowly and with visible effort usually needs more practice at the current level first.
See our guide on teaching math when you're not a math person — staying one lesson ahead covers most of what's needed at the elementary and middle school level.