The shooting happened around 12:11 a.m. in the 500 block of Dover Street in Joliet, police said.
Investigators said a 5-year-old boy was shot by an 8-year-old child Friday at a residence at the location. The boy was taken to a local hospital and then airlifted to Comer Children's Hospital in critical condition.
The 8-year-old child had gained access to a handgun in a bedroom of the home, Joliet police said.
The Joliet Police Department and the Illinois Department of Children and Family Services are investigating.
No further information was immediately available.