Mine is three, four in August--Endless Alphabet, Sight Words, JumpStart Preschool 1&2, SuperWhy, Tracing ABC, and Kids Number and Math are the ones we like on Android.
The Ipad has these and more. Before having a kid, I had heard of Team Umizoomi and dismissed it as garbage. For all its silliness--Team Umizoomi was what got her adding and subtracting. By three she was counting 100 and by 3.5 she was doing + and - with anything under 20. The quirky things that make parents happy. :)
The tablets help with forming letters, sounds, and now the sight words. But nothing beats real books. We read to her every day, by two she was pretending to read on her own. There are cheesy princess and mickey/minnie books that are rated learning to read--one or two really simply sentences a page. Not sure whether she has memorized the books or learned the words--but she rips through them now. New books she can read about three in four words on the 1st pass through.