I took the tour with my dad last year. (we're big kids- i'm 30ish hes 60ish.)
It was interesting but i wouldnt take a real young kid- i think they would be bored- my son was only 2 so it wasnt much of a decision onyhow. I seem to remember an age limit of like 7 or8 - so i guess they already figured that out.
YOu go on the very first run of the train in the morning when they check out the line, they share alot of the railroad's history with you and you get to go backstage to the 'barn' where they keep the trains, right next door to the monorail shed.
One other thing- it's EARLY - quite a bit of time before the park opens. For some reason they did it on the EE days (not relevant right now but maybe in the future) and even then we got in while everyone else was still waiting for the gates to open (you meet a conductor outside the gate and he lets you in). It was august and i remember leaving OKW before sunrise! We had a rental car, i'm not sure you could make in time from anyplace besides a monorail hotel without one, you would need to hit the very first bus of the AM to have a shot- if you're late the train has already left.