There is an itunes application for ck12,
http://www.ck12.org/.
Someone else sought out math programs for my ipad, which I have not used but I can name them for you.
Telling time
Math Gr5 Lite
Math Ninja
Math Series
HyperBlast
BForceBlaster
My First Clock
Super 7 HD
Jungle Coins
numPyramid
Drills
MathBoard
WolframAlpha
Fractions
Math Fractions
MMillionaire
MathAcademy
HyperBlast2 HD
Math Magic
Rocket Math
Math Party
Motion Math
Math Bingo
Mathemagics
Grade 4 Math
Math Ref
Grade 2 Math
Training
Place Value
I really like the timed drill work you can get on CK12. I've only tried it out on the computer, not an ipad.
I am actually planning on checking out a remedial math program at a university that was designed with cognitive psychology,
http://www.mun.ca/mlc/home/. The side bar has a section "research and innovation."
Certainly if you look at how math learning and development happens, one thing is certain, we've gone too far away from drill work and ensuring students know their math facts. It was pointed out to me that students in high school often don't know their single digit addition math facts. They can figure it out, but they don't know it. Their brains are going through the same process an ESL student has to go through to understand what is being asked, before they can actually work on figuring out the question. The goal should be that students can do 60 single digit math questions in a minute, and about the same for multiplication when they are at that level. If they don't know their basic math facts like they recognize colours, they will simply have a lot of problems with math later.
I'd say the other thing is you want to do as much as possible to use the technology to enhance their learning, not to use it in a way that they rely on the technology to do their math facts.
Good luck.