Try text-based adventure games:
My first interactions on a computer was with a few text-based adventure games where my brother and I would have to read a paragraph and then type out some simple commands ("look floor") to get the next hint. Because English was not any of our primary languages, we spent a lot of time with a dictionary as well. Graphical adventure games (ooh, colors!) came later and still required a lot of reading and typing.
The entertainment factor of the game provided plenty of motivation to get over the hurdles of reading, understanding, interpreting, and trying solutions.
With this background, my first thought would be to google
ipad adventure game and look for games that are suitable to the age range. many of the highest-rated results were actually top-10 lists with many promising titles.
If I were you, I'd look through those titles and consider these factors:
Does the game require reading?
Obviously you will want to choose games that have written output (see a random example below) so you should avoid games that have spoken output.
How much reading is involved?
You will want to pick a game that has lots of text and not just the occasional word thrown in. For your purpose, the text needs to be the central carrier of the story - like a comic book. (Hey, there's another idea: comic e-books?)
Does the game require typing?
Depending on the challenge you want to present, a game requiring commands to be typed can be a great way to improve typing and thesaurus skills -- but perhaps games based on typed input are past era now?
Many games have a point-and-click interface meaning that you can press a command from an on-screen list and then press the on-screen object you want to use with that command.
Who is the intended audience?
Some games are made for kids, some for teens, and some are decidedly adult. Make sure to check the description, and try the game yourself before you let Junior play.
I don't have an iPad so I don't really know what's "out there" but I'm sure that there's plenty to choose from - Lego Duplo, Smurfs, Marvel, and much more.