I've seen a very successful interactive math talk to a participating audience of five and six year olds at MSRI a few years ago. It was structured around the question of "what's the largest number?" Kids had fun coming up with large numbers but eventually one of the youngsters figured out a simple way to always come up with a number bigger than the previous one named. Through some nice leading by the teacher, they eventually concluded (indeed proved) that there are an infinite number of natural numbers. It was fun watching the kids (by then fully engaged) grapple with what is really a quite abstract idea --- that one can know that the numbers are unending without actually exhibiting them in any way. It was a terrifically sneaky way to engage kids and hear their ideas on notions like "infinity" or even "number".