Just downloaded this iPad only app which went from $2.99 to free yesterday. M has been playing with it today. It is a nice little storybook that has interactive elements and really sharp graphics. On each page, the reader learns quantity by tapping the number of objects that are on the page. Then there is a simple animation when you tap the right quantity.
The one problem I have with the app, is that the reader can not swipe to the next page until you do the tapping activity and the animation is finished. On the other hand, it does reinforce the concept of sequence.
Grab it while it is still free!