I'm making the assumption that she gets her regular mathematics lessons in school, and that is why you are asking about software rather than books. (Please correct me if that assumption is not correct.) A good set of books can be challenging and fun even if it is not on a computer. I like Miquon Math

http://www.keypress.com/x6252.xml

for starting out my children, and then the Singapore Primary Mathematics materials (which now have an edition aligned to United States curriculums standards)

http://www.singaporemath.com/Primary_Mathematics_Stds_Ed_s/1...

appropriately supplemented by ALEKS

http://www.aleks.com

and EPGY.

http://epgy.stanford.edu/district/info.html

(The last two of these resources are online services with adaptive learning software.)