Here's another take based on the feedback. My 14 yr old is loving this and doing a lot of the planning and all of the drawing. He's the oldest. To address some of the concerns or invite better suggestions:
There is no door to the room, it is open to the right of the basement steps. It is probably going to be a crawl under. Old people, like myself, can choose to stand in the entrance.
I appreciate the comment to make the benchwork as tall as possible!
The boys are 7, 10, 12, 14 years old.
I might have liked a swing or lift bridge but I'm willing to crawl to preserve the usable space. If I get desperate I have seen layout with telescoping legs and I believe I could orchestrate lifting that whole section toward the ceiling, like an elevator.
The outside loop is O-72. The inside loop is O-60, the reverse loops in the original were O-72 and O-48. In this revised version he matched the reverse loops at O-48s. He put them off the outside loop in order to maximize space. He managed to get the reverse loops away from the timesaver. We do want to be able to reverse the train for interest. He does question cutting the middle in half but thought it would give another option. We're going to lay things out on the carpet and see if we can stand in the spaces before benchworking. He forgot to switch pens on this one, so it looks flat, but the intention is to have the inside and outside loops completely separate. The inside loop will be at least 6.5" lower than the outside loop.
We have a good number of operating accessories that we intend to swap out. Culvert loader and unloader (one of those is the manual version - manual is more fun!), a manual coal loader, log loaders, milk car platform, magnetic cranes, icing station, horse corral...
The more trains the merrier, but we wanted at least two They are good about having multiple on a track. Many of what we have can use a remote and if you use finesse you can run conventional and TMCC on the same track. The main way to protect the trains, is train the kids!!! They generally prefer to have someone driving and someone else loading, unloading, coupling, uncoupling, etc. The driver gets to be the boss, and who doesn't want to boss their brother?
Previous to this I have background in electronics and programming and have rigged up occupancy sensors that cut the track power to sections or to the whole layout if a collision is imminent. I may work some of that sort of thing into this, but mostly for "friends" and such.
Thank you for the great and continued suggestions.