I have just fired up BVE4 and showed my little brother RJ it. I fired it up and loaded the DLR Bank to Lewisham route and showed RJ it... he looked interested - asking "what's that", "look at that train". I told him that if he presses ENTER it'll sound the horn, so everytime we passed a train on the route he pressed it on his own accord. Then I told him that to change the reverser to forward he needs to press UP and then to power the train he needs to press Z.
He learned that quickly and within a few minutes he ended up knowing to wait a few seconds after the doors closed and he knew to up the power and turn the reverser to forward to go. Bless... he is only 3.