Have you studied the rail map of Switzerland, with the scenic routes highlighted?
https://www.mystsnet.com/en/download/swiss-travel-system-map-en-2023/?wpdmdl=14294&masterkey=63a023c8da36c
If you want to ride the Glacier Express to go from Chur to Lauterbrunnen, you have two options as to routes. Either way I suggest you break up the long journey with an overnight somewhere.
Option 1: Glacier Express from Chur to Andermatt (ride the actual GE for this; otherwise you will have to change trains at Disentis for the rack railway section). At Andermatt, take the train the short distance to Göschenen , where you can pick up one of the many trains traveling the Gotthard Pass route toward Luzern. You can either stay on the train (probably changing at Arth-Goldau to get to Luzern), or leave the train at Flüelen and ride a boat the rest of the way to Luzern. This will take a bit longer but the scenery is beautiful, and the boat is fun because the kids can run around a bit. My family (with older children) did this in September and everyone loved it.
Overnight in Luzern and the next day take the scenic Golden Pass route to Interlaken Ost, changing there for Lauterbrunnen.
Option 2: Stay on the Glacier Express to Brig, changing there to a train going to Kandersteg for your overnight. This is pretty little mountain village and better for an overnight than Brig. Next day continue to Spiez and then to Interlaken, etc.
I suggest this instead of continuing to Visp, because the trains between Visp and Spiez spend most of their time in a very long deep tunnel. You can see this on the map—-tunnels are indicated by dashed lines (not dotted lines). There is still a tunnel on the way to Kandersteg from Brig, but it is much shorter. And you will likely see some of the car-trains on this route (trains with flatbed cars carrying passenger cars through the tunnel). That would be fun for your train-loving child to see.
