Recently I made some trains from Lego for wooden rails. They looked pretty familiar so I compared them to my old H0 trains and ho and behold they are extremely close in size. I quickly made a bunch of different wagons and trains. This way the kid can play with his H0 push along trains on his wooden railway network. Doing the same on my proper H0 railway resulted some catastrophic failures. If he breaks the Lego trains, I will be able to reassemble them with ease. Plus they are pretty cheap. The V90 set with all the wagons cost only 30 Euros.
So far I made a DRG Class 80 steam loco with a matching coach wagon and a DB V90/100 with a flat wagon, a cooler wagon, a generic freight wagon and a tanker wagon. I'm in the process to make DB TRAXX and maybe a V43. I used solid bogies with those big wheels. The pin allows enough movement for the wheels take regular turns. The Traxx will have rotating bogies because that is freakin long, 27-28 studs.
More pix here: https://bricksafe.com/pages/Pendra37/4-wide-trains