With 6,6, and 9, You are probably going to enjoy the free bus tour much more than the Up Close tour which costs extra.
The Free tour basically uses video's on the buses between stops, and has a few stops which you can take your time at. It's last stop is at the Saturn V center which is very cool.
The Up Close tour is a bit more indepth and takes you behind the scenes a bit more. It also includes an actual guide which can make it a much more interesting experience for those who have questions. When I went in November it including a stop at the VAB and we were even able to go inside the VAB (not allowed since the mid-70's) where we were able to see one of the shuttles getting refurbished and readied for Museum duty. It also included up-close drive-bys of both Apollo/Shuttle launchpads, which is much closer than the stop the free tour makes. It ends at the Saturn V center, which then allows you to catch the regular bus back to the main vistor's complex.
I loved both the Up Close Tour, and the Then&Now tours I took in November, but with the 3 kids and the extra costs, I'm not sure if the kids would enjoy those tours nearly as much to make the extra cost above the included one worth it for you.