Teach them how to make a game of some sort. Something really simple like hangman in BASIC would teach them a few different things all at once and still have a fun result.
Would you really recommend BASIC? That's what I learned in, but I really feel like Python is just as intuitive -- and if they stuck with it, useful.
One option would be Scheme with Bootstrap (http://www.bootstrapworld.org/). It's a middle school program, but you could probably strip it down for third graders.
Why not JavaScript? It requires no command line knowledge and runs in any modern browser. They can create games, easily install libraries or frameworks by including another JS file, etc.