Twitter wasn't just on display. It served a real function. The audience WAS the conference goers. You can't really use it as a comparable example in this case. I seriously doubt a social network aimed at kids will achieve even a fraction of growth from TC50.
Whats important here is that the type of people that will pay attention enough to go to tc50/demo aren't just going to go there and then not say anything about it.
They'll come home, get on their macbooks and start blogging what they saw ( some really nice programmers building a kids network ). Some of their friends will check out the links, maybe share it on facebook, myspace, or , gosh, in an email. And so forth...