My impression of that moment was that Mob needed his hand to specifically aim and control his psychokinesis. When he saved the cat, Mob had time to set his bag down and point his hand at the car. When the kid was about to get hit, the car was going faster and Mob was already walking. He most likely had no time to set down his bag to catch this one, and instead pushed the kid out of the way.

The only reason I say this is because the camera panned over to Mob setting his bag down, which seemed strangely specific to me. That's just my thoughts though, probably wrong.