What you say depends on the ages of all the children in your household.
Older ones do watch Newsround, do talk about what they've picked up (and for the big global stories, this includes stuff from the playground). I see it as my job to be truthful (I wouldn't, for example, say 9/11 was an accident, but if I could omit cause altogether, I would for a younger child). And I always would say, from a very early age, that things a reach the news because they are rare and exceptional.
This is why traditional fairy tales and other stories with baddies have such a resonance. Children need to learn that baddies exist, that life isn't all under our control; but that disasters are rare, it's OK to feel sad and worried, but we can cope. If the adult is unflappable, the child will learn confidence even in adversity.
If someone had said this to me when my eldest child was 3, I'd probably have thought them insane and that it was way too young. But by the time my youngest was 3, the dynamic had moved on completely.
BTW - by year 6 (age 10/11) mine were discussing AQ with flair. It might seem like a loss of innocence, but it's not a new phenomena - in my day it was IRA bombs.