One of the sadder holiday traditions is Colombia's annual Christmas campaign to persuade homesick guerrilla fighters to sneak out of the jungle and surrender their weapons.

Over the past three years, Lowe SSP3, working with Colombia's Ministry of Defense, has decorated huge Christmas trees in the jungle, floated LED-lit plastic balls filled with hopeful messages and sent small gifts down rivers the insurgents use to get around. It's also lit the sky with beacons to provide a guiding light for guerrillas escaping camps at night.

But this year they are bringing out the heavy artillery: the guerrillas' moms. The "Mother's Voice" campaign tells the real-life stories of about 30 mothers whose children are members of the Revolutionary Armed Forces of Colombia, better known as FARC. Each poster shows a real family picture of one of today's armed fighters as an innocent child, with the mother's message: "Before you were a guerrilla, you were my son." Ads end with the line, "This Christmas, we're waiting for you at home. Demobilize."

To shoot the TV, radio and poster ads, each mother was asked to bring photos of her child and send a message asking him to return to civilian life.

"We always talk to [former] guerrillas, and one thing that worries them, especially now that the peace process is advancing, is that they may be rejected by their families," said Jose Miguel Sokoloff, Lowe & Partners' chief creative officer. "Mothers are the ones who will always forgive us, but we don't always realize that."

These campaigns, in which jungles and rivers become media channels, have unexpectedly become awards-show juggernauts. The "Rivers of Light" effort involving LED-lit balls sent down the waterways won a Titanium Lion at last year's Cannes Lions International Festival of Creativity as well as Grand Prix awards at ad shows around the world. That project was inspired by former fighters explaining that Christmas is important even to a revolutionary, and the simple insight that living in remote areas forces travel on water rather than by road.

In 2012, the agency learned guerrilla fighters were being shuffled around to prevent desertion and make it harder to find their way to towns to surrender. That led to "Operation Bethlehem," with the beacons lighting a path to safe destinations. Glow-in-the-dark billboards along the rivers said, "Guerrilla, follow the light." Mr. Sokoloff said about 200 guerrillas did so.

This month FARC announced a ceasefire starting Dec. 15. That means guerrillas will have more time over the holidays to spend in towns with access to TV and radio, making for an easier media schedule. "We want them to think about what they're missing," Mr. Sokoloff said.