We just completed Grade K using Sonlight (Core A) and RS Math A. We completed RS A and are a ways into B since DS was accelerated. Yes, we skipped to the starting point in B past A reviews, but the review is a VERY fast-paced and a condensed version of A. For a pre-K child I would be very hesitant to start with B. You may be able to overlap and combine (with some effort), but simply learning to use the abacus takes time, as do the basics which provide a foundation for the program. If you do decide to try to use B for ease, the levels of the two seem incongruous enough that it would be worth the effort to try to find a used version of A (at least as a backup) if you find you need help expanding on certain concepts/slowing down for your youngest. Good luck! It is a WONDERFUL program and worth the time it takes to teach it.