The victim, Hazmeera Clayton, was struck near her throat at the Montgomery Gardens complex where a block party was going on. She was taken to Jersey City Medical Center, where she is currently in grave condition.
Police, who arrested Thompson Saturday afternoon, say the little girl was not the intended target; however, they are investigating whether the incident is gang-related.
The housing complex, an area plagued by criminal activity, is near the St. Peter's college campus.
The weapon has been recovered, cops say.