Commissioning a museum computer exhibit can be a confusing business. This page has links to documents written by a range of experts. At the moment a lot of this advice is from people I know. But if you know of some other good advice on the web let me know and I'll add a link.
A Museums and the Web paper by Holly Witchey. She discusses what children like about museum web sites and tests some out. The kids don't mince any words - "It's not even close to exciting...".