RSBM
I work for Children's Services in NY, not CA, but with that said, CA & NY parallel each other on many things social-welfare-related.
It's not so black and white as you present; for starters, there's a huge difference between an accident and abuse/neglect. On any report of suspected (neglect, abuse) there must be an assessment of child safety and the family's risk factors. The family's history is reviewed, their strengths and supports are evaluated. If it is determined that the child isn't safe or if there is imminent risk to his/her safety, then the parent(s) will be asked to help make a plan for the child to stay with (family, friends) while we assist the family in resolving whatever is putting the child at risk. If there are no suitable resources for the child or the parent/caretaker is unwilling/unable to assist in planning for the child, that's when Foster Care/out-of-home placements and changes in custody come into play.
Parents make mistakes, and sometimes those mistakes are pretty egregious, but it doesn't mean we automatically snatch their remaining kids and terminate their rights. People have to be given the chance to improve themselves and parent appropriately. I can't say that in my 30+ years of history here that I've ever even heard of a case involving a child being harmed in a situation like this, but I've seen plenty of other bad accidents that involved serious injury and didn't rise to the level of a removal (it's mostly lack-of-supervision stuff, like severe burns from boiling water or catching the curtains on fire, or loss of an eye or limb from motorcycle/4-wheeler/snowmobile accidents, or accidental drownings, etc.) Although each instance is heartbreaking and many times disturbing because it could have been prevented, we still have to look at the big picture before taking someone's child.
If you're asking can it happen at all, ever? Sure, things could certainly be bad enough in the home that we would have to immediately take and place the remaining children to keep them safe. Does that happen a lot? Again, it depends. The decision to remove a child isn't made in a vacuum, there are many, many things that factor into it before it's presented to a judge.
I know, it's frustrating to people who have specific expectations for parents and ideals as to the proper way to love and cherish a child, but the reality is not everyone parents the same and that's allowed. "Helicopter parenting" is a popular term and many people push for parents to control ever minute bit of a child's existence with the goal of keeping them safe, but then I think about my own upbringing and realize that I wouldn't be the person I am today if I hadn't had the opportunity to accrue the injuries and scars that came my way. We can pass judgement on people all we want, but it also means that someone is passing judgement on us and we may not live up to their expectations for how we (parent a child, treat our dog, mow our lawn, and on and on...) It is what it is. The important thing is that at least this group has learned from this tragedy and can maybe take steps to share those lessons with others and prevent it from happening again.
Edited to add my MOO...