Here's another cartoon creator (thanks, Kate!), this time from Chogger.com, with my first go with it in the screen capture, above.
You've got the sort of interactive tools you'd expect — text (including speech bubbles and captions), images (including drawing them and importing them), the ability to add new panels (3 shown above, the "active" one in white) and so on.
It isn't entirely intuitive if you're 50-plus like me, but to a kid familiar with technology this one will be a breeze.
You don't get the hundreds of ready made cartoon characters you do get at similar sites like ToonDoo, but I actually think that's a plus, as it leaves a lot more up to the user's imagination and creativity.
I particularly like the fact that you can import photos (in the screenshot, I've imported my SecondLife avatar). You (and I really mean your learners) can tell stories with speech bubbles in PowerPoint but I think young learners will love this one as an alternative.