The killings came amid growing resentment against the fighters who seized the capital in March and took control of the government. They are roaming the streets and, human rights monitors say, killing people including a priest and a lady carrying a baby.
The17-year-old was among 64 children who had been demobilized from the rebel group known as Convention of Patriots for Justice and Peace (CPJP). They were brought to the capital in December for their protection after a new rebellion was launched in the country's volatile north, UNICEF said. He was later recruited by the rebel alliance known as Seleka, which was created in December and ousted the president in March.
The boy and a second former child soldier who had since turned 19 were killed by a mob in late April after they reportedly stole a vehicle on the orders of a local rebel commander.
"Action must be taken against those who are recruiting and using children to commit crimes," said Souleymane Diabate, the country representative for UNICEF.
Even before the latest rebellion, UNICEF said more than 2,000 children in Central African Republic were with the myriad of armed groups destabilizing the country's north.
Child soldiers were used in some of the heaviest fighting in the battle for the capital. They directly engaged troops from South Africa in and around Bangui from March 22-24, according to South African soldiers who survived the fighting that left 14 of their comrades dead.
On Friday, Human Rights Watch said it had spoken with witnesses who detailed summary executions including of a priest who was shot while calling for calm with a Bible in his hand.
"A woman with a baby on her back was walking down the street past the bridge, when she was shot by a Seleka fighter and left dead in the street with the baby crying on her back," one witness told Human Rights Watch.
The watchdog also documented the case of nine men who had been abducted by Seleka forces in April and accused of being members of the national military. The men were repeatedly stabbed by rebels before being taken to a river bank.
"After arranging the nine men in a line, the Seleka soldiers shot five of them," Human Rights Watch said. "Their bodies fell into the water."
The new government set up by the rebels has blamed much of the recent violence on militias loyal to ousted President Francois Bozize and on fighters who are not officially part of Seleka.