Have you looked at Dreambox? My son is in second grade and has been doing it for almost a year and enjoys it (although it can get challenging). I'm pretty sure it starts at the K level. IMO, it's a great supplement to a full math program.
Dreambox used to be available at a discount through the Homeschool Buyer's Coop. I'm not sure if it still is or not, but it's worth checking out.
If you go directly to the Dreambox website, you can also sign up for a 2-week free trial.