Totally agree with ampere. It's far to easy to blame SS. The fact is, there are people in society who want to neglect, abuse and hurt their kids.
If they really want to do it they will often find a way.
There was another v similar case reported on at length in the Guardian a few weeks ago, a little girl called Tiffany who starved to death in a locked room above a pub while her mum joked with punters downstairs. It beggars belief that any parent could do this, but these cases come up with depressing regularity.
I think there's a real disconnect on MN when it comes to SS - we're forever advising each other to ring SS and report a worrying neighbour, or child we know, and saying 'better safe than sorry' but in fact whenever it's any of us being called upon by the SS the instant and universal reaction is 'tell those bastards to fuck off, how dare they question you, you're a wonderful mother, don't they know how cruel this is when you're already down' etc etc.
We see other people as needing SS, but take it as the absolute worst insult when the SS might want to speak to us. The parent in this story told the SS to sling their hook, as I can only imagine many parents do - they'll be well used to it.
SS are demonised and seen as evil, interefering baby snatchers, not as people who can provide support etc and who need to make sure that people's kids are ok. That will never change in any meaningful way will it, so SS will always have a really uphill battle getting access to vulnerable kids.