We went last week, and I was really impressed with the whole experience. If your children enjoy science centers, etc. I think they would probably enjoy this. Please note that I'm not that into space stuff and the last time I had visited was in 94. Back in 94, I found the tour super boring and said to my husband, "I can't believe how bad this tour is. They could do so much more with this." We'll it's really good now, 17 years later.
We only spent a partial day there. Slept in, had a relaxing morning at our condo, and drove out there starting at noon from Orlando. It takes 1 hour and 20 minutes to get there. The main attraction is the tour that is included with admission with two stops. One is a structure you climb up and view launch pads. The second is the vehicle assembly plant where you see how big everything really is. This second stop included two short excellent films. One was about the 60s (A little about politics and the space race with the Russians, clips with hippies, models, etc. from that period too where really fun and my son really got a kick out of seeing that.) Next was a short film about the Apollo program and mission. This was shown in an area that housed real control units from that era. These were educational and entertaining. I loved them.
After spending time at both these stops we went back to the visitor center area, and some in our party did the space launch experience and others just visited the Rocket Garden. If we had been there earlier we could have attended a 25 minute talk with an astronaut, seen an IMAX movie, etc. We, though, were really happy with the activities we did in our short time there (about 4 1/2hours) -- felt this worked out really well for us and were glad we made the day trip.
We didn't think the crowds were bad at all considering that this was a busy spring break week. We only waited about 10 minutes to get on the bus for the tour, not bad at all. And isn't wasn't wall to wall people at all. Sure there were plenty of people there, but just like a typical day at any popular science center.