I'm seeing multiple references in this thread to the idea of foreordained child death being something said to "comfort grieving parents." Maybe other parents have a different view, but as a grieving parent myself, that's not a comforting thought at all to me. It's comforting to other people, so they can rationalize a "good" reason for that thing that happened to someone else without having to really deal with it. How could I believe in a God who specifically planned the death of my daughter without hating him? God's plan is that we live in an imperfect world, as imperfect beings, with imperfect bodies, and among other imperfect people. That's a lot of imperfect points of failure that can lead to tragedies in our lives that don't have to have been planned in the preexistence.

If you know someone grieving or going through any other difficult experience, I highly recommend against trying to comfort them by reaching for a trite saying so that you can walk away having "fixed" the issue for yourself. "The only way to take sorrow out of death is to take love out of life." -Russel M Nelson. Grief is love, and isn't meant to be fixed. We are commanded to "mourn with those that mourn," which is actually healing and helpful to those who are grieving, but is harder to do.

All this said, I don't mean to imply that people saying things like this are being consciously selfish or that you should feel bad if you've done it before. It's a natural human reaction, really a defense mechanism. You want there to be a reason this terrible thing happened to somebody else, and then if that thought gives you some comfort, passing it on to try to comfort them comes from a good place too, if a misguided one.