If you've never been to Kennedy Space Center (KSC), the standard bus tour (included with regular admission) is sufficient. I've done two of the three current extra tours. (Note: I don't have kids, nor do I know any 6 or 9 year-olds, so I have no concept of what would be interesting for kids of that age.)
KSC Up-Close Launch Control Center Tour
This tour takes you inside one of the Firing Rooms where rockets and the Space Shuttle were launched. As with all KSC operations, the tour guide was extremely well-informed. The tour itself, however, was fairly disappointing. The main area where technicians/scientists sat had been completely cleared of equipment. (KSC is a working facility where rocket launches happen frequently.) When you enter the Saturn V Center on the regular tour, you will see a re-creation of the Firing Room. That's more interesting than seeing the real thing, which turned out to be a open area with lots of power outlets. (Note: This may have changed since my visit.) I wouldn't recommend this tour...even at the reasonable price of $25.
KSC Up-Close Cape Canaveral: Then & Now Tour
This tour takes you onto Cape Canaveral Air Force Station to visit some of the early launch sites. You'll see examples of 50s/60s technology. Or, lack there of. Seeing the primitive tools used, it's truly astounding what was accomplished. The tour also includes a visit to Launch Complex 34. That's the site of the Apollo 1 disaster where astronauts Grissom, White, and Chaffey lost their lives. I was born in '72 and I found this tour fascinating. I'd bet that anyone who was alive during the early space race might be even more interested in seeing some of the history that they saw on TV. I would have been okay with paying a lot more than the $25 that they charge for this tour. (Note: The KSC website says this tour is currently unavailable due to operational constraints. They plan on re-starting the tour once the sites become accessible again.)
Some General Admission Tips
Do not miss visiting the Atlantis facility at the main visitor center complex. The way they display Space Shuttle Atlantis is pretty fantastic. Although you may be tempted, do not bypass the "pre-show" movie presentation. It's worth the wait.
I'm a annual passholder to KSC and every time I've visited, the Atlantis complex has had docents stationed near the shuttle. Take advantage of those people, as they are (mostly retired) technicians and scientists who spent years working on rockets and the Shuttle program. On my last visit, I asked a simple question that lead to about 40 minutes of incredible stories told to me by a man who worked on the solid rocket boosters. I probably learned more from him than I did on the entire Launch Control Center tour.