There is quite a gap between the first car that goes by the kid and the cars IN FRONT of the first car. That car appeared to see the kid because he was going slower than the rest of the cars and very purposefully went low to avoid the kid. The gap between the first car that went by (slowed car) and Stewart’s car was not that great. And Stewart was going faster than the first car and higher up on the track than the first car. Leads me to believe that Stewart may not have been able to see the kid, whereas the first car to go by did. That is speculation of course.
The kid ran towards the oncoming cars, so if the other drivers were looking ahead to the stopped car, where of course the driver would be, they would not anticipate a driver on foot in the middle of the track far before the wrecked car.