I love RightStart for PreK-3rd grade.
First of all, the books keep their value, so you can get A and resell it if it is too easy or rush through A as a review and then move on to B.
For most Kindergartners (who are actually 5 or 6 years old) I recommend Level B. They do spend time learning match facts to 10, but just not as much as in Level A. You can slow it down if you need but Level A would be too slow for most kids at that age.
I would never recommend B for a 4 or young 5 because it quickly gets in to more advanced concepts.
Don’t be afraid to take a full year or a year and a half to do Level B. It covers a whole lot of information that will help kids be more successful in years to come.