Rick, I don't know how to draw or post pics which is why you will see on my posts I try drawing "word pictures" and usually end up offering what I hope are helpful suggestions to layouts others have drawn. I'm thinking the solution for what you want is really 2 layouts with a couple of connections.
I think from previous threads that you are looking to run f-units, gp units, & switchers, all 4 axle power. If you limit yourself to 4 axle power, and resist temptation to buy modern, long chassis dash 8-44cw, sd70macs, etc. then you can run on 18 inch radius, and perhaps even a 15 inch radius on some industrial spur tracks.
I would do the mainline on the 4x8 table. You could use a "folded figure 8" for the mainline. Essentially 2 36" x 40" loops that go up and over or down and under each other for the mainline. This would give the kids the maximum mainline operations for them to run the trains on. Do some mountain scenery, with a river accross the middle at a slight diagonal long ways. Maybe some mountains along the pit on the back of the 4x8 section. This would allow for bridges and tunnels, all the things kids really like to watch trains go through, over, and under.
I would shift the "operating pit" to the left so that the "bridge on the right is 30" wide while the one on the left is only 6 inches wide. Make the 6 inch wide bridbge a hinged lift up for access. Your back will thank you! Put a switch on the lower tracks at each end of the mainline going over the bridge on the left and to the back side near the wall on the right. Put in insulators with double pole double throw switches so that you can operate the layout as 2 layouts.
Now you have space to design a switching layout in a "L" shape 8' x 30" with a 18" x 30" "L" extension on the right side. the switching layout should have a double track mainline that comes off of each of the switches coming off either end of the 4 x 8. This will alow an "in" lead and an "out" lead.
Install 2 control panels. One throttle will control the mainline from outside of the 4 x 8 section in the bottom of your picture. The other panel will be in the operating pit, facing the switching section of the layout. You will also need at least one dead end spur track that has a power routing switch, or an insulator with an off switch.
Now I'll describe how to operate it with the grand kids. They will run a train on the 4 x 8 until they start to get bored. When they start to get bored, have them slow down the train at the facing point switch onto the switching section. Throw the switch and bring their train into the switching section. Your switch engine is already parked on the spur with the power off. As the train crosses the gap, you take it over from the industrial yard panel. Drop the caboose off the train just before you get to the switch that goes from the single track transition to the double track main through the industrial section. After you cross the switch onto the double track portion and once the last car clears the switch, you drop the rest of the train on the empty track, and move the locomotive on past the other single track to double track switch. Throw both of the switches on both ends of the single track to double track sections, and hook up the locomotive to the new train you made up while they were running on the 4 x 8 mainline. Continue backing after hook up to pick up the cabboose you dropped earlier, and then take the train back out on the 4 x 8 main on the other leg for the grandkids to run the new train. You will now have a bunch of new cars to spot at the industries. You can also use this to teach them to share. You give each child the chance to operate 5 laps around the 4 x8 or 10 laps around the 4 x 8 or whatever numbers you set. Then you bring the train to the switching section, change the cars, and the next one gets their turn to run the 4 x 8 mainline for an equal number of laps.
One last suggestion that I forgot, but it may be obvious. If you put mountains and tunnels on the back side of the 4 x 8 leave "window" opennings to the operating pit so that you can rerail any cars that come off the tracks in the tunnels. Put guard rails on all bridges and in the tunnels, or maybe even use Atlas rerailing sections exclusively in the hidden tracks in the tunnels to help with railing cars or rerailing some that have derailed. Also a grade crossing in the switching section with an Atlas rerailer will make it easier to put the engines and cars on the tracks.
