I've done a little of this with 9-12 year olds. The most important thing is quick feedback and keeping it simple. They absolutely loved making websites. I taught them enough HTML to get started and set them up with a editor and preview window. I wrote a little web app for this, so that they were editing a "live" site they could show their parents when they went home (or keep working on it, on another computer). It was super simple and just had the functions they needed. Also, as someone else suggested, using the inspector tool to mess around with existing websites was really fun for them.

Another project was using arduinos -- and I even got them writing C, which more or less worked. You didn't need too much code to blink a light and they loved that. The mix of coding and the hands-on wiring up their breadboards kept it engaging. We did a little project where the light blinked with a frequency related to a temperature sensor, so when you put your finger on it, it would blink faster. One kid had the grand idea of having it blink faster when cooler and slower when warmer, and watching him figure out the math was great.