After Arithmetic?

I believe that they will be fine in Pre-Algebra, as it looks like the skills learned in it will build on the skills learned in Arithmetic (for example, the fractions lessons from arithmetic should help with ratios and proportions from Pre-Algebra), but it depends on your child. For the most part, though, Arithmetic should allow your child to tackle Pre-Algebra, because Pre-Algebra is mainly about strengthening arithmetic skills that will later be needed for Algebra.