There was a Cambridge policeman stationed at the door of Aidan’s school this morning.

At first I was relieved to see him. Then, while Aidan was dutifully wiping rain off his shoes, I glanced nervously at the gun in the cop’s holster not three feet from my son’s head and got nervous, wanting no gun, not even a policeman’s, anywhere near my son.

On my way out of the building, I wondered if he would stay there all day or if he’d only show up for drop-off and pick-up, whether he’d only be there for only a few days to reassure parents or become a permanent fixture, a sign that security measures are shifting in the wake of the tragedy at Sandy Hook Elementary School.

Since Friday, everyone has been talking about school security—on the news, on facebook, at dinner parties. (A knockdown drag-out erupted and as quickly dissipated on one of the mom listservs.) We’ve been talking and arguing and posting and tweeting about disbelief and rage and sadness and about gun control and mental illness and having the balls to change legislation for both. The president gave a moving speech. People everywhere have offered their prayers to the community.

And I have been lying awake at night thinking about the families.

The families whose lives were ripped apart on Friday never to be put back together. The parents who went out on Friday morning to shop for Lego’s or Barbie dolls to put under the Christmas tree for their kids, only to find out, in the instant that would change everything, that their children would not be coming home.

Those parents are not thinking about gun control or mental illness. They don’t give a shit about gun control or mental illness. They don’t need our prayers. They aren’t interested in how to avoid the next tragedy of this kind.

They want their children back.

Those families are never going to recover from this. They are going to spend everyday of the rest of their lives grieving for their children who were senselessly wiped from the earth, the children they loved more than they ever imagined loving anything, their sweet, beautiful children, who will never drive cars or go to college or have children of their own.

I can’t stop thinking about them, except when I’m thinking about the kids in Aidan’s classroom. There are 20 of them. That would be every single kid in his class. I keep picturing it. I can’t help myself. I wondered when I saw that policeman this morning if schools are worried about copycats. Oh my God, it’s too much to bear.

And then I think, Is this the day that the most horrible thing conceivable happens?

Part of me has been waiting for it to happen since Nina died. I’ve always been terrified by horrible, sudden tragedies of this sort that just make no sense. I blogged about it when Natasha Richardson died in a skiing accident. But this preoccupation intensified when Nina died, when I became a person who knows firsthand that the worst can happen.

Then again, part of me has been waiting since Aidan was born.

Parenthood is treacherous. From the moment you bring that tiny, fragile person into the world, you risk losing him.

Bob Dylan said, “When you ain’t got nothing, you’ve got nothing to lose.” And when you’ve got everything, it can be ripped out of your hands.

No warning. Just gone.