Nicole Hockley and Mark Barden find the work of Sandy Hook Promise emotionally excruciating, but they will not let up

VIDEO: Parents of Sandy Hook Elementary Honor the Boys They Lost by Teaching Other Kids How to Prevent School Shootings

Mark Barden carries a smiling photo of his 7-year-old son Daniel and readily shares the boy’s too-short story.

For Nicole Hockley, talk of her own son Dylan – just 6 when he, like Daniel, was killed in the Dec. 14, 2012 massacre at Sandy Hook Elementary School in Newtown, Connecticut – is more of an emotional minefield.

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“I never allow myself to cry in public because I don’t want people to focus on the tears,” she tells PEOPLE in an interview for this week’s issue. “I want them to be focused on what I’m saying, and what they can do to not be in my shoes.”

That’s what Hockley’s and Barden’s lives are all about now, three years after gunman Adam Lanza, 20, shot Dylan and Daniel in their classroom, along with 18 other first-graders and six educators: Doing everything they can to prevent another school tragedy like theirs.

Say Something

Just weeks after losing their sons, the pair walked away from their respective careers (Hockley, as a marketing executive, and Barden, as a musician) to form, together with other grieving parents, an organization they named Sandy Hook Promise.

After exhaustive research into threat assessment and the behavioral-science studies of school shooters, they developed the “Say Something” program that, since it launched in September 2014, has trained more than 400,000 students in 350 schools on what they can do to prevent violence in their classrooms and corridors.

Hockley and Barden, assisted by a growing cadre of volunteers, lead training sessions for kids as young as second grade (and as old as high-school seniors) on how to be more connected to their peers, recognize warning signs that someone might be planning violence, and then tell a trusted adult who can help.

Barden tells PEOPLE Now that “Say Something” also works to overcome kids’ natural resistance to what they think of as tattling: “We tell them that when you’re snitching, you’re trying to get someone in trouble. But when you say something, you’re helping them.”

“Kids leave the training feeling empowered because they’ve never really thought about before that they are seeing and hearing things – particularly on social media, on Yik Yak, Burn Note, Kik, all the sites adults are not on – that the adults in their lives are not,” says Hockley. “That suddenly reinforces the message to them that they have the power to do something to help protect their friends.”

Saving Future Lives

Among the identified red flags kids should look for – on social media and in everyday interactions – in their peers and then confide to a trusted adult for intervention are: extreme mood changes, negative role models, bragging about access to guns, fascination with images of death or suicide, and warning a friend to stay away from school or an event.

Dr. J. Reid Meloy, a forensic psychologist, says that 15 years of research – by himself and by the clinical psychologist Dewey Cornell, Ph.D., along with the FBI, Secret Service and Education Department, has found that the majority of school and workplace shooters leak their intent to a third party in advance. So, Meloy tells PEOPLE, “that kind of intervention that Sandy Hook Promise is teaching will save lives.”

And that, says Hockley and Barden, make their work worth the pain of steeping themselves in the issue of school shootings after their lives were shattered by one.

“I can’t rewrite history,” says Barden, who shows Daniel’s photo – one of him, at age 5, squeezed close together and and laughing with his older siblings, James and Natalie – as part of the “Say Something” trainings. “I tell the students this was my little boy, not just a number or a statistic as part of this epidemic, but my little Daniel, and you need to know a little bit about him.”

“I can’t bring my little Daniel back, but what I can do is prevent other families from having to live this pain and that absolutely keeps me going – to honor Daniel and protect the lives of other children.”

Says Hockley: “It is hard. And some days it’s impossible. But my commitment is to give Dylan a legacy and save lives in his name, and to build a better future for my other son, Jake. And I just don’t want other parents to experience this and know there was something you could do to stop it.”