Before Swift Playgrounds launches to the general public, Hopscotch and ScratchJr are still two of the best apps for children to explore learning to code on the iPad.

https://www.gethopscotch.com/

https://itunes.apple.com/us/app/scratchjr/id895485086?mt=8

For children who are older or more advanced:

Codea https://itunes.apple.com/us/app/codify/id439571171

For more of a game-like environment:

Lightbot

https://itunes.apple.com/us/app/lightbot-programming-puzzles...