We took our six and seven year olds, they enjoyed it. Not nearly as much as we did, but they enjoyed it.
Here is the thing with Cirque and little kids.
Its pretty expensive. If the idea of paying $50 or whatever for your kid to see a show is OK with you, consider going. I was too cheap to pay that until this year.
Although one of the least abstract and conceptual shows Cirque does, its still sort of abstract and conceptual. Probably won't make a difference because its colorful enough and filled with enough action for a kid.
Watch potty breaks. I'd make sure my kid was going to sit 90 minutes without a potty break before I did Cirque. Too expensive and too good to miss because "I haffa go!" This was the big reason we didn't do Cirque until this year. I've missed five minutes out of lots of movies due to potty breaks, and when you are watching a stupid movie for $6, that's ok. (I can't remember if La Nouba has a "no reseating if you leave" policy. That is common for Cirque shows, but La Nouba is a little different because of the number of small kids).
It isn't scary, my kids weren't impressed because at six and seven they had no idea how difficult the things done were to do, no real understanding of the strength and flexibility the performers were showing. They did enjoy the show though. The other thing to be aware of is that it might give certain kids ideas (probably ones older than four) for dangerous stunts to try on bikes or trampolines.
They do expect small kids and even provide booster pads so you little one will be able to see!
(We, on the other hand, were blown away - as is typical with a Cirque show).