Union Station, Kansas City sponsored one last year. Don't know if it is an annual affair, but it was very interesting. Got wide regional participation and looking at what different kids/teams were doing was very interesting.
We were volunteering for another organization that day, so didn't get to see everything. All in all, a very enthusiastic group of kids.
it is a huge event (at least the SF Bay Area flagship faire), but your experience on a smaller scale faire depends on the local "maker" interest on how big it will be.
If you like mythbusters, then this is the type of event for you.
There are definitely lots of activities, demos, and booths targeted for kids. There is also a degree of alt culture/steampunk and the such.
If you go to a big event prepare for a density of nerd sweat especially for the indoor areas when Adam Savage or other famous person is giving a keynote speech.
Hmm, I see that there is one in NYC, long drive, but I sorta have a shot at free tickets. Too bad I'm suffering from a cold, which may or may not be gone by then. But I think there is a mini one at MIT next month. I might have to put that on the calendar.