The child appeared to be uninjured, according to the source and witnesses.
The chaotic scene, with gunfire erupting near the heart of the U.S. government, brought a swarm of emergency vehicles and caused Congress and surrounding offices to be temporarily locked down.
Police also closed Pennsylvania Avenue in front of the White House.
CNN's Athena Jones, who was at a Senate office building near the Capitol, said she heard gunshots that sounded like fireworks.
The FBI dispatched units in response to reports of shots near Garfield Circle, which is on the Senate side of the Capitol.
CNN Chief Congressional Correspondent Dana Bash learned that one person was injured. No further details were immediately available.
House and Senate sessions were immediately suspended.
A Capital Police bulletin said reports of gunshots required "all occupants in all House office buildings to shelter in place."
"Close, lock and stay away from external doors and windows," the bulletin said.
Authorities later lifted the lockdown.
President Barack Obama was briefed on the situation. An intelligence source says so far, there is no tie to terrorism.
Arkansas Congressmen, including Mark Pryor, Steve Womack, Tim Griffin and John Boozman took to Twitter during the shutdown to report they and their staff were safe.
More updates from CNN.