Put it in terms they understand, like this:

Copyright is kinda like, the "right to copy" something. When you type a book report for school, you can print as many copies as you want, because it's yours. Maybe you want to print extra just in case you lose one before you hand it in. Maybe you want to print a copy to give to your uncle Joe the English professor to proofread. The sky's the limit for you because you wrote it. But other people can't pick up a copy from Joe's desk and make a hundred photocopies or scan it and place it online for every 5th grader to hand in as his own work, and your teacher can't copy it and publish it in the school newspaper...without your permission...for which you should charge...and give me a percentage!

I like the snow shovel analogy, since it's easy to understand but allows for discussion of many of the nuances and tradeoffs:

http://www.econlib.org/library/Columns/Mcchesneysnow.html

When it snows hard in Chicago, people have to shovel out parking spaces for their cars. But no one will want to do this if someone else can just take the space as soon as they leave. So the rule (not a law, but socially enforced) is that the person who shovels the space "owns" it for a limited time. This means that people can do useful work and be rewarded for it. But their "property" returns to the public after the snow melts. (Think about how the outcomes would change if the "ownership" were just for one day, or if it were forever.)

This doesn't capture all the interesting things about copyright (like the non-scarcity and zero marginal cost of information goods) but it does capture some important differences between "intellectual property" and real property that get left out of most discussions. And the Chicago parking system is controversial just like many copyright issues.

Or maybe...copyright is a way to make sure that the guys who created the product get as much money for it as possible. Why make it a moral issue when its more like a cultural issue in the way humans have implemented resource sharing and creative compensation.

I'd start with moral rights - most kids seem to have an idea about not letting people copy their school work. Copyright means someone else isn't allowed to take your homework and hand it in with their name on it.

Then I'd talk about maybe an artist or musician and how they earn money to do what they do - ignoring the part about the record companies. Mention that it is the state that helps ensure, with copyright law, that the creators of works aren't ripped off, that the state protects them for a certain amount of time so they can get paid for what they do. Book writers is a good example for why a longer period of time is needed, people can spend years writing books, if someone can simply copy it then the author can possibly only sell one copy.

If they catch all that I'd progress on to how large corporations have perverted copyright so that they can get the most benefit from a creators work (but that creators may still make more than they would because of the companies marketing abilities) and then how copyright terms have increased and that has changed the whole deal stopping works entering the public domain [as early].

Well, http://www.copyrightkids.org might be a starting point. It seems a bit hokey, but might give you some ideas.

It's put together by the Copyright Society of the U.S.A. I really don't know anything about the organization.