We use Scratch and find it very accessible for pupils. I tend to teach them the basics, and begin with an animated story, once they have mastered that I show them the useful stuff (collision detection, timers, variables etc) I then use the tutorials at learnscratch.org - Home to show them the basics of making a simple game. These tutorials have the scripts attached so for differentiation I get lower abilities to modify a game.
Our Year 7s love it!