Alex Sawyer was charged with two counts of murder and four counts of aggravated arson.
The fire injured eight people, including three firefighters.
Friends and family gathered at Byrd Park Friday to remember 2-year-old Ju'shaun Watson and 4-year-old Justice Stewart.
The boys were asleep when their apartment caught fire. They suffered smoke inhalation and were airlifted to hospitals in Chicago, where they later died.
Authorities did not elaborate on the motive.