Social services have an extremely difficult job. I know several people that work in the field and they are easily the most dedicated, conscientious people I know. They are damned if they do and damned if they don’t. On one hand they get blamed for not intervening quickly enough, on the other they are accused of being over zealous, of interfering and breaking families up. They certainly do not take incidences like this lightly and of course there are investigations with such cases, but short of monitoring people 24 hours a day, you can never ever guarantee that something like this won’t happen again.
As others have said, there’s only one person guilty of causing that child’s death, and he’s in the right place.