The UK Coastguard said on Wednesday it was assisting with a "large-scale incident involving up to 70 people" in the Mourne Mountains, Co Down.
The group of cadets were on an annual camp based at Ballykinlar when they got into difficulty.
Reports have suggested that some of the children are suffering from hypothermia and leg injuries.
The coastguard said it was contacted by the Police Service of Northern Ireland (PSNI) and ambulance service just before midday.
Also on scene is the Irish Coastguard Rescue 116 helicopter.
A Coastguard spokesman said: "Three Coastguard helicopters, three Coastguard rescue teams, police and ambulances are involved'".
The Coastguard said the three helicopters are now situated at Donard Park due to poor weather conditions and will support casualty evacuation as further information comes in.
There were up to 70 young people involved, in a single group, although some of them became separated from the others, he said.
Three helicopters are at the scene and the emergency is understood to involve a group of around 40 young British Army cadets who were camping in the Mournes when they got into difficulties due to the weather.
The MoD said 43 cadets and 10 leaders were in the group and several needed treatment for ankle injuries from slipping on stones.
Mourne councillor Henry Reilly said conditions are "unbelievably bad for August".
"I'm down in the lowlands so in the high Mournes it must be incredibly hard".
One woman said on Facebook: "I was coming over the mountain from Rathfriland at approx 11 am and weather was wild".
Emergency services are on their way.
Colin McGrath, an SDLP MLA, tweeted: "Major emergency incident in the Mournes with helicopter, coastguard, medical and ambulance responding".