No, I don't think you can ask that and not sounds heartless.
This always happens after a tragedy involving children, people come on here and ask "well why... blah blah blah" to make themselves feel better and like that would never happen to their child because their parenting is superior.
The fact is that we are all one decision away from tragedy and then scrutiny from strangers online.
The kids were out, playing in the snow, having fun, as kids should.
The parents will be going over and over what they could have done differently, and they will do this forever, long after all the 'innocent' questioners have forgotten all about this tragedy.
I know from when my son died, I literally stood in a shop listening to two women gossiping about his death, it was so far from what actually happened that I didn't even recognise it was me and my son they were talking about until they said the street name, but their words have stayed with me for over 2 decades now.
If you're alright with making a bereaved parents pain even worse by asking questions that are, frankly, nothing to do with you, then go right ahead, but don't pretend it's for any reason other than to make yourself feel better or more in control.
Bereaved parents come on here for support all the time. They should have nothing but sympathy. The rest of the whys and wherefores are nine of our business.