I have not taken my kids yet, but did got to this a couple of years ago for a trip that was just DW and myself. I have two daughters, one 5 and one 8, and I think they are both old enough to really be mesmerized by the show. So we are thinking about taking them during our next trip in October. The DVC discount currently does not extend out to Oct, so we may wait before booking tickets.
My guess would be that when you starting getting to the 4 and under crowd, they may get spooked because it can be dark and loud at times, but who knows, really depends on your kids.
We took our kids 3 years ago to see it, they were 2,7 and 9 and they all really enjoyed it! Well ok they enjoyed 85% of it but they were ready to go at the end!