Im not denying negligence, however from reading reports the parents/boyfriend was totally coniving fuckers. It was hidden from social services that the boyfriend even lived there.
Whether they knew/should have known the boyfriend lived there is immaterial. They should have spotted the kid was covered in injuries and was being abused. That's their job. To fail to do the job on such a massive scale reeks of incompetence on a scale beyond mere cock-up and strays into the area of criminal negligence for which all concerned should be investigated and if necessary prosecuted.
Actually, I'd like to gut everyone involved, including the sorry fuck of a "mother" and her evil male companions, with a rusty knife and leave them to bleed to death while attempting to scrape their own innards back into their sorry bodies, but I understand the law doesn't currently allow for such a punishment. So I'll settle for a criminal negligence prosecution.