It depends a lot on the age of the kids of course, but as @jaakk said, play. It's hard to get kids to be interested in something if you're just sitting them down and trying to explain it to them. Get hands on. For the purpose of this answer I'm going to assume you mean "design" in the general sense of the word, not just web design.

Show them how design is responsible for every day things that they're already using. Take them to an industrial design museum, perhaps a conference that focuses on 3D printing so they can see with their own eyes how everyday things are created from scratch. Next time you drive past a stop sign, ask them why they think it's red - that alone opens up doors to discussions about everything from psychology to evolution.