Continuity mistake: In the airplane, the window shade in the seat next to the main character repeatedly opens and closes between shots.
Continuity mistake: After the old folks turn into kids, Harry, the boy with the glasses, is seen climbing up the side of the house, yet in the next shot, he is running through the bushes toward the camera.
Revealing mistake: When Passenger asks Driver to pull off the side of the road, so that he can "show him something really scary", Driver pulls way off to the left shoulder, instead of just pulling onto the right shoulder.
Revealing mistake: As Car Driver approaches the yellow caution sign of the winding road they are traveling, he continues driving straightforward, never once turning the wheel, and periodically turning his head to talk to Passenger.
Revealing mistake: As Mr. Connor flies into the retaining wall after being thrown from the Vietnam setting, you can see the "stone" structure wobble.
Answer: It's unlikely we'll ever fully understand. However, there were already a number of violations involving the children, prior to the stunt. It seems to boil down to the audiences at the time demanding more and more dangerous stunts and actions in their films that the film makers, and Landis, tried to accommodate. Landis also seemed less concerned about the dangers and either didn't think it would be that risky or was more concerned about finishing production on time. For whatever reason, Landis ignored warnings of the dangers. But given that he violated night time production laws involving the children, including hiding the children from welfare workers and telling them to keep everything a secret, shows he was more concerned with getting the shots and must have felt the stunt was that important.
Bishop73