I wouldn't necessarily start with the kindergarten level. There is a placement test you can do to see which book would be best for your DS. If your child gets a certain percentage correct, then they recommend the next level up.
http://www.singaporemath.com/Placement_s/12.htm We got primary math 1a text/workbook for our DS4 after having him do the test. We have just been doing it occasionally when he shows interest, and we finally got to some new stuff for him and he started liking it more (he doesn't like doing stuff he already knows how to do.)
Good luck with the homeschooling!
Since we haven't done a whole lot yet, I can't give it a full evaluation. But it suits us for where we are now.
