The two were alone in the car when the 4-year-old found Brayden's mother's unsecured gun and discharged it, fatally striking Brayden, the sheriff's office said.
In 1927, Congress passed a law barring the USPS from mailing concealable firearms unless they were from licensed dealers in an effort to curb crime.
A 19-year-old student who is charged in an Arlington Heights school resource officer gun theft appeared in court and was ordered to remain held in custody.
Two people have been charged in connection with a north suburban school resource officer's gun that went missing earlier this week.
Classes have been canceled Tuesday at Forest View Educational Center in Arlington Heights after a school resource officer lost a firearm in a restroom, the school said.
Trump's Republican administration alleges the ban violates the Constitution's Second Amendment right to bear arms.