I have had good luck with a small table that is my grandchildren's. 24" high. I use trains that I don't mind repairing and anything on the table they can play with. I recently had to add a log loader and dump car for the grandson, who is 5. He gets confused still on the order to do things. Fun to watch. I started them around 2 with Thomas and a loop. LionChief. He was starting to get bored.
They can run my trains under supervision. Legacy and LionChief.