Police said 26-year-old Charles Matthews, who is homeless, was charged with assault, endangering the welfare of a child and criminal possession of a weapon.
The attack happened around 7:00 a.m. Tuesday inside the Bushwick Houses near Bushwick and Flushing avenues.
Police said a 9-year-old boy and his 16-year-old sister were waiting for a bus inside their building when a man approached them and punched them in an unprovoked attack.
The boy was punched from behind and his face hit a window ledge, causing a huge lump to swell on his forehead. His sister said she was hit on the side of the head.
"At first, he was pacing around the building, cursing, yelling at his self, like, you could tell there was something wrong with him," the 16-year-old victim said.
The kids called their mom in tears, and she took them to the hospital.
Residents of the Bushwick Houses and surrounding community gathered at the scene of the attack later that day to call for the immediate arrest of the assailant.
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