R.J.
Oh! We LOVE Cirque :) :) :) and have been taking kiddo since he was around 4 as well.
What we do (before every show) is to go to their website and watch clips of the show that we're going to. If we look at other shows, we make a big deal of the fact that they're different AND hype the one we're going to. We also kept up a running dialogue during any point where it was allowed (seating, during audience participation, clowns, etc.)
It will be great fun. Enjoy yourselves. And remember: Circus, not opera, even though it's posher than Ringling Bros. :) :) :)