For kids I'm a fan of http://dragonbox.com/ http://dragonbox.com/ and their http://www.dragonboxapp.com/ http://www.dragonboxapp.com/ They do a pretty good job

For kids I'm a fan of http://dragonbox.com/ http://dragonbox.com/ and their http://www.dragonboxapp.com/ http://www.dragonboxapp.com/ They do a pretty good job of teaching symbolic algebra to kids who don't yet know arithmetic.