The entire Duck Duck Moose collection of kids' games, with an educational focus, has been made free by the self-learning website and is downloadable now on Android and iOS.
Most of you might know Khan Academy as a site similar to Coursera, Udacity, or Udemy. It's similar, but while those are more all-rounders, Khan Academy has focused more on basic academia. You can go there to learn algebra, art history, or macroeconomics. There are coding courses too, but for that you're probably better off with one of the previously mentioned sites or Codecademy.
It recently brought game studio Duck Duck Moose under its wing though, and that's great news for parents because it means all of those games are now free. They're all popular and highly rated. There seems to be more available for iPhone and iPad, but there's still about 11 to choose from on Android, such as Itsy Bitsy Spider.