There are Splash Math apps for both the iPad and iPod ranging a wide span of grade levels. Some are free and some are not. They seem to change from time to time. I would recommend checking several of them out either way because even if your grade level app is not available, I am sure you have students that could benefit from the one designed for the grade level higher or lower.
The app covers a wide range of mathematics skills. It is not in game form. It also provides a scratch pad in case the students need to work out a problem. Lastly, you can set the device to email reports of their progress in the various mathematics skills.