The officer comes back and tells you the car was searched but no clues were found.
And then you find out that he did not even look in the trunk.
What would you have to say to the officer?
"Hey, don't feel bad. You gave it the ol' college try. At least the parents can't sue us. Come on... I'll buy you a dough-nut."
Nope, I think I and every other thinking person would ream the heck out of him for his incompetence. And I hope the officer involved would feel every bit as horrible as he should for his role in what happened. If he didn't he doesn't belong on the police force and donut jokes aside I still think the vast majority of cops are pretty decent people (subject to some notable exceptions) who are hurt when things like this happen.
All of which is a long way from finding the police legally responsible for these childrens' death even assuming the truth of the fact pattern you posted. There still is a lot that does not make sense here and I am not sure any of us have the facts to make any kind of judgment. Maybe the report will shed some light on things, although I am sure its accuracy will be questioned.
Just a horrible case.