We have had good experience with Dreambox. They cover the more arithmetic parts of the K-4 curriculum (this week they finally introduced 4th grade core stuff, plus fractions and time in the lower grades).
The interface is audio (if reading is a potential issue), it is adaptive (do not provide direct help!) and the graphics/animations are appropriate for the lower ages (much more polished and cuter than MathWizz). Since it is a paying service they don't have ads. It works well on Safari and Firefox on Mac. Since it is Flash-based it won't work on iThings (but the parent dashboard will).
My kids like it. The transition to the much drier interface of Aleks was tough for my oldest
Biggest issues are that it doesn't cover the full state curriculum but that would not be an issue for a 2yo, and there is little inside the program to do speed drills on tables (my son was getting frustrated when the program started to assume, after teaching the concepts, that he had also aquired the automatic fast replies). We used xtramath.org (free, no ads) for drilling.
http://dreambox.com
