Well, a trip the Johnson Space Center is a must. However, the place is over price! and if you have kids, they are going to want to eat there (You have no choice)
The first thing you want to do as soon as you get there is to board the tram and get it out of the way. It can get very frustrating, not only because of the long lines, but also, because some of the staff members that run the place are very rude and unfriendly.
When you are done with the tram, you are free to get around the place, there's plenty to see and do there...