The web-based version of Scratch2 would be great on an iPad, but unfortunately it uses Flash.  MIT say that they are working on a version for the iPad, but it does not seem imminent.

In the meantime you could use Blockly on the iPad. Blockly is derived from Scratch and runs on the web.  As it uses just  HTML and JavaScript, it works beautifully on an iPad.  It is simpler to use than Scratch and does not include all the Sprites that come with Scratch. There are different subsets of Blockly blocks that you can start with including Puzzle, Maze and Turtle.

A structured programme of tutorials has been created at learn.code.org.  Here you can register and work through the tutorials over time.

Blockly is also a great way of introducing javascript as you can see the javascript code for your program.

To start using Blockly have a go at blockly maze on this page

To learn more have a go at the tutorials