DragonBox. It's a game of match and eliminate symbols according to some basic rules, e.g., remove the same from each side. Usually around level 12 is when you realize the rules are algebra.

http://www.gazziliworld.com have some great educational games

[shameless plug]

We build collaborative educational games on the iPad. You should check out Equator (https://itunes.apple.com/gb/app/equator-2-player-problem-sol...).

I don't have specific apps to recommend right now. But you can find children apps reviews at these sites:

http://momswithapps.com/ http://childrenstech.com/

search maddie and matt on the apple store. its free