HI,
There is a rather steep learning curve for the new user ...
Let's start at the beginning...
Routes:-
This is the layout - think of the model railway set you had as a youngster (?) Basically it's the track and some scenery and some signals and speed limits.
It can be as small as a simple oval, or something like the Canadian Rockies route very long with super scenery.
Session:-
Before you can drive a train, you have to put it somewhere on the track. That's a very basic session, but gives you the idea. Again, a Session can have lots of different trains with AI drivers, and instructions for each one.
There's also the Surveyor - "Create a Route".
That's the tool that lets you create a route...
I know it's a bit frustrating, but try the Tutorials (maybe without your son?) to get a feel of the controls (ignore the steam engine controls!).
When you feel a bit more ambitious, try the Kickstarter routes - following the instructions carefully!
When you're feeling ambitious, Create a new route - select a simple track and make a simple circle, place a loco on the track. Now save the route and default session. leave surveyor, select your route. Now try driving the loco (as fast as possible!).
You'll find that most of the routes will have speed limits (even the ECML - a UK route where the Passenger train speed limit is 125mph - out in the countryside). Exceeding a speed limit is going to vcause problems...
Alll the Best,
Colin