A 14-year-old boy who flashed a gun while holding up a children’s lemonade stand last week in Boston was charged with two counts of armed robbery, the authorities said.

The crime outraged people across the city as it struck at a rite of summer for many children: opening a roadside lemonade stand, complete with powdery mixed drinks, plastic cups and a till full of cash.

The boy also faces two counts of unlawful possession of a firearm, according to the Boston Police Department, which arrested him on Friday in South Boston, the neighborhood where the robbery occurred on Wednesday. The authorities withheld the boy’s name because he is a minor.

The boy and an accomplice, who as of Monday was still at large, had been casing a lemonade stand that two siblings, Juliette, 11, and David Byrne, 12, were running on a street corner after school, investigators and family members of the siblings said.

When the pair finally approached the siblings, they asked whether they accepted Apple Pay before grabbing a bright pink lockbox with cash in it, police said. Tiffany Byrne, the aunt of the two children working the lemonade stand, said in an interview last week that about $80 was stolen.

One of the suspects then “displayed a black firearm in his waistband” before the pair fled, according to investigators, who released home security camera footage from the neighborhood that showed the suspects wearing masks as they ran off with the lockbox. Some of the images showed them with their faces uncovered.

“What happened to my son and daughter shouldn’t happen to anyone,” said Dave Byrne, Juliette and David’s father.

Mr. Byrne spoke on Friday evening about the violent encounter during a block party at the same corner, where hundreds of people, including city leaders, showed up to support the reopening of his children’s lemonade stand.

“It’s just horrific and unacceptable that something like this could happen,” Kevin R. Hayden, the district attorney for Suffolk County, Mass., said at the gathering.

Investigators were urging the public to share information about the second suspect in the armed robbery.

The 14-year-old who was arrested was expected to be arraigned in Boston Juvenile Court, police said.