But there is a difference in the picture. You'd also determine that it's going left when looking at the windshields, as only the left side is completely straight.
Honestly, this image is a terrible example to illustrate their point. I think it's more likely that children are more likely to actually use public transit, which is why they'd be able to detect the missing door quickly.
The right windshield is straight as well, you see part of a building behind it
Kids immediately recognize the view must be from the driver's side of the bus because they're not examining the situation. They're taking the first piece of relevant information and rolling with it. No door means passengers get on the other side, so it's going whatever way you assume traffic across the street should go based on where you live.