Well I must confess I am not a parent, but I am a VERY INVOLVED Aunt of 3 nieces and a nephew. My oldest niece is 6 and has always had an interest in the planets and stars, so I would take her there this year if we were going down together, but I think anything under that might be pushing it. The others are 5 and under, and I wouldn't take them since they would be overwhelmed and probably not very interested.
I think your 6, 8 and 9 year old boys will love it. Depending on what Nasa is doing too, they can view the launch pads, maybe see a shuttle, depending on where it is at the time you visit. They have great I-max movies that aren't that long, a shuttle for the visitors to walk in and out of, a control room, you might be able to see some of the payloads that they will be sending up to the Space Station, there's a whole building with all sorts of Apollo rockets, banners, memorabilia, moon rocks. Very cool stuff. And if the kids have the slightest interest in this stuff, they will be even more interested by the end of the visit!
The one thing I'm disappointed in is that we never got over to KSC to see a launch. The one time we had tickets, they canceled the launch and the shuttle didn't go up until after we went home. Last I heard, since 9/11 they no longer let visitors on the property to view the launches, although I am told there are spots you can drive down to on Cocoa Beach and see it, and we DID see one from Kissimmee once, but it would have been cool to be closer.
Have a great time!