Continuity mistake: In the subway toilets, Argie checks two cubicles and looks in the third one. However, in the next shot, he is looking in the fourth cubicle.
Continuity mistake: In the scene where Billy Crystal is chasing his grandson at the recital, he has day-glow paint on his face. A few seconds later in front of the stage, the day-glow paint is gone.
Answer: It's demonstrated throughout the film that Artie struggles with the modern way of doing things. This may extend to modern car seat belts with their five point harness systems. That and the fact they're already late for school and holding up the traffic makes it a stressful situation for Artie and it was probably easier (and to add comedy) for Artie just to rip the seat out with the boy still in it.
Neil Jones