I'm totally with the "acknowledge their interest but don't let them dominate the group" model. A lot of what other people are saying is more specific to how to handle a kid who wants to ANSWER all the questions so I'm just going to point out that as far as the kid wanting to ASK all the questions and contribute all sorts of cool stuff goes, that's a good time to acknowledge, "Hey, I can tell you're really excited and want to talk about these things. Why don't you come up afterwards and we can get into it some more then?" as another form of letting them know you're not ignoring them but also redirecting their energy to a more appropriate time.