The teens, many of them students, work 12-hour days for half of what adult coworkers earn, sources say.
North Korea merely reaffirms an earlier commitment to denuclearizing, with no specific timetable.
US President Donald Trump and North Korean leader Kim Jong Un make history by meeting, but doubts hang over any nuclear deal.
Authorities want to prevent possible disorder inside the isolated country during the key talks in Singapore.
Controls by China relax after Kim Jong Un and Xi Jinping meet in March and May.
The letter comes five days before US President Donald Trump meets with Kim Jong Un in Singapore.
State Security agents are pressuring family members to convince their relatives to return home.
Authorities are watching citizens for signs of prohibited money and movies from the South.
The domestic propaganda campaign includes phrasing that indicates Pyongyang will not give up its nuclear capabilities.
Meanwhile, most North Koreans are unaware the talks were even cancelled.
Gone With The Wind, with its themes of civil war and hardship, is a particular favorite, one source says.
Few believe summit meetings with rival South Korea and the US will improve their lives, sources say.
The warning comes after the North calls off talks with South Korea over US-South joint exercises.
An unknown number are killed or injured when a blaze blamed on faulty wiring sweeps through a workers' barracks.
US Secretary of State Mike Pompeo secures their freedom during an unannounced trip to the North.
High-rise apartments are being offered in Sinuiju city for purchase and investment opportunities, sources say.
A former North Korean diplomat discusses why Kim won’t agree to an irreversible denuclearization.