Nothing structured. Lots of counting and grouping, classifying and thinking skills work. Most of which is accomplished by playing with legos and counting their goldfish crackers (or whatever, you get the idea). I'm a delayed academics kind of gal, so for pre-k, I go really low key.

If you really want workbook style stuff, I can recommend Kumon without qualms because that is exactly the kind of thing they do. Fun counting games and getting comfy with numbers. As for true curriculum, RightStart Math has a wonderful manipulative/game based program that I think is phenomenal, if labor intensive for the parent (not recommended for the mom teaching lots of kids at different levels). I would not truly recommend it until AT LEAST age five or six though. Hope that helps.