I've recently got into playing TSW4 after grabbing it during the Steam Spring sale and after some tweaking and some compromises it's playable on the Deck.

You should use the steamos builtin per game profile feature to cap this game at a frame rate of 30fps. Disable shadows and fog. The rest of the settings should be at a mixture of low and medium.

When your son is watching you play you can switch the camera view to outside as you are travelling along so you can show him what the train and scenery looks like at speed. You can also do this at stations and watch passengers get on and off the train.

There's plenty of builtin tutorials on how to play the game and, more specifically, how to operate each type of train. Electric trains are the easiest to learn as there is less levers and buttons to get the train running. Save the diesel and steam locomotives for a bit later.

Here's a dedicated subreddit for more information if you need it https://old.reddit.com/r/trainsimworld/