So what exactly happened to the

It was never addressed in the films. Ned Benson (writer/director) mentioned it in an interview last year.

Talking about the exact thing that happened, I noticed – at least in Them – that it’s not specified how their child died. Was there a reason behind that?

I didn’t think it was important. I think it’s funny that that’s one of the things that frustrates people, that we didn’t tell you exactly what happened. But is that really important? The story is about the relationship, and I know exactly what happened to that child and so did the actors. And so did everybody involved in the film. And the closest I get to it, and I allude to it in a very sort of analogical way, but it’s the moment where she is releasing that firefly and she asks it to wake up. And you know, the child died of crib death.