We were just there last weekend with our soon to be 7 year old. Great advice on booking your tours as soon as you get there. Honestly, the only part of the tram tour my son really enjoyed was the rocket park portion where he could get out and walk around the Saturn V. He liked the shuttle and the 747, but that you don't need tickets for that. A lot of Space Center Houston is videos and seeing exhibits. There are some interactive portions, but not as many as you might think.
Astronaut training was not avaialble for a reason that escapes me. I just don't know how much a 7 year old would appreciate old mission control, even though it is cool.