There are no passing sidings on the upper loop. You can only run more than one train if all are going the same direction. If you did use a reversing loop for one, you would have a head on crash. Add two passing sidings or lose the loops.
The lone passing siding on the lower loop looks to be about 4 feet long. That's a very short train. Again, this is a single direction railroad. If two trains are running opposite directions, one is stopped at the siding while the other is moving. Two longer sidings will provide much more operating interest.
Planning is more than drawing tracks on paper. You/your sons need to test whether the train movements they want to be able to do are possible on a given design. Some folks can do this in their heads or on paper. Others need to assemble the track on the floor and put some cars on it.
I recommend you lay out your plan on the floor with track, one level at a time, and have your sons push the cars around by hand. Have them redesign it by changing the track. Can each level keep two sons busy having fun?
When they are happy with each level, then you need to test whether the two are compatible. For example, can 5 people fit? Do they get in each other's way? Can anyone actually get into those spaces inside the reverse loops? You might cut a full size paper mockup of one level and put it under the actual track of the other. One pair of boys is pushing actual train cars, the other pair simulates their actions on the paper track. Operation of accessories can be simulated as well with a piece of cardboard the size of the footprint and the operator.
This whole planning and design activity will be a lot of fun in itself as they see what doesn't work, come up with ideas to fix the problems, change the track, and see if their ideas work.
BTW, they won't be boys for much longer.
