A+ Math Success in 30 days: Addition app for iPhone and iPad

Based on the profound principle: Slow and steady wins the race

1) Start with manageable steps.
2) Having mastered that step, take a small step forward.
3) Keep moving forward without taking a break... and success is sure to follow.

This app starts out with extremely simple and manageable steps
Day 1: Add by 1
1+1
2+1
3+1
...

2) Having mastered adding by one, you move to day 2.
Adding 1+2 to 5+2 (not 1+2 to 10+2). Remember, simple manageable steps.

3) Day 3: 6+2 to 10+2

... you get the drill.

Practice 40 questions a day. You can set the pace for you child using the settings menu. However you can answer no more than 3 questions wrong within the time allotted to move to next day.
On completing a day successfully, you get a checkmark against that day. This helps keep track of progress and provides motivation.

An extremely simple principle indeed!
Yet more effective than any other approach you can take to acquire a new skill.