Innocent childhoods are being "smashed to pieces" by the civil war in Syria, Samantha Cameron said after visiting refugees fleeing the conflict.
The prime minister's wife said she was horrified by stories she heard from those caught up in the violence during her visit to Lebanon as an ambassador for Save the Children.
She welcomed the work of a clinic for new mothers and pregnant refugees run by the aid agency in the Bekaa Valley and a specialist centre helping children deal with emotional trauma.
"As a mother, it is horrifying to hear the harrowing stories from the children I met today. No child should ever experience what they have," she said.
"With every day that passes, more children and parents are being killed, more innocent childhoods are being smashed to pieces."
Among the youngsters she spoke to were a 10-year-old girl who described seeing her mother die in the rubble of their shelled home and a 13-year-old who dragged her uncle's body from the street after he was shot.
Mrs Cameron - who has three children - also heard from a mother whose three-year-old son was shot by a sniper next to his six siblings as they fled Syria in a car.
Save the Children chief executive Justin Forsyth, who accompanied her on the visit, said: "Samantha Cameron's support as a Save the Children ambassador helps draw attention to the plight of children caught up in this terrifying conflict.
"Without more help, and quickly, we risk losing a generation of Syria's children."
Former foreign secretary David Miliband warned that non-intervention would lead to serious long-term consequences.
He told Channel 4 News: "There is a very big choice to be made. I'm afraid it's not about small arms because in the end Syria is awash with small arms. There is a much bigger choice that the US, Britain, others have to address.
"I think the prime minister's instinct is that you have to do something big not small but he is obviously feeling very, very constrained at the moment."
He added: "It seems to me as an outsider who doesn't have access to the military dossiers or intelligence anymore that if there is to be Western intervention it needs to be something that shifts the balance because my real fear is that non-intervention means this Assad regime is going to go on killing people for two or three years and all that we fear that about the collapse of the country - even if the Opposition eventually inherit - is going to be there and Syria will dissolve in front of our eyes.
"If the state structure of the heart of the Middle East collapses my God, we are going to be dealing with the consequences for a very long time."
He conceded the legacy of the Iraq war did make it difficult for British governments to act.
"There is a due humility because more has gone wrong as a result of the Iraq intervention than has gone right but if a consequence is that we end up with more Bosnias then we can't be learning the right lessons."