J.O.
I just raved about this on my personal facebook page and always see this question on here - Agnitus - it's a free app loaded with tons of educational games. My 3 1/2 year hasn't put it down and chooses it over her pixar movies. AND IT'S FREE! - you can also get access to reporting to see where your child is doing well (numbers?) and where they struggle (letters?) - this feature is an in-app purchase but well worth it if you're interested... but it's not required if you just want your kids to play. Oh - multi-kids (multiple profiles) can use it so you can track their progress separately too.
Search - Agnitus - perfect for 2-5 year olds. ((they are coming out with more games supporting kids up to the 3rd grade next year so keep your eye out))
http://www.facebook.com/agnitus