Generally the apps mine wants to use (about to be 5) are youtube videos of toys and playing games. When my daughter was a little younger she liked this one a lot, it's shape identification, words etc. A company called Toca Boca and another called Dr Panda make nice toddler game apps too:

https://itunes.apple.com/us/app/i-see-ewe-preschooler-word/i...

I don't have much faith in the (esp. autonomous) educational value of any of these apps, but we have gotten pretty good results learning how to sit at the kitchen table and do kindergarten homework, activities, board games etc.