Elementary Pro for Android
Can be also used for learning basic english words for foreign kids.
There is a scoring system that you can use to keep track of your kid progress and recognize achievements (e.g. ask your kid to get 50 equations right).
Also - quite a lot of fun for adults to check how many simple equations they can crack within a minute ... feel free to report your records in the comments :)
This is an ads free version of the app. I recommend you try first the ad supported on called "Elementary Free" in a try before you buy fashion.
More games to come in the future.