It's not in North America like these suggestions, but I would recommend visiting CERN in Geneva. I visited there a few weeks ago, and I was impressed by their public exhibit about particle physics. It would probably to children around 8 and up, I figure.

Edit: Oh, and while you're in Switzerland, go to Berne and visit Einstein-Haus, where Albert Einstein lived when he developed his Special Theory of Relativity. It's not quite as accessible to children, but still interesting.