OP I have been in a similar situation, and I did report the family in the end. I was put off and put off by various people for various reasons, all trying to fix things and telling me to stop meddling, while the child got worse and worse. I also tried to help out, but it didn’t do anything except make the parents neglect worse because they knew someone else would pick up the slack feeding the child, nurturing her, washing her clothes etc. I was also concerned because the mother was pregnant, and I doubted her ability to look after a baby and the older child, as she wasn’t managing the older child alone.
In the end I reported because the mother assaulted the child in my home. The step father threatened to kill me (told mutual acquaintances) . I offered to call the police and luckily that put him off and they have stayed away from me since. SS were round their house within 48 hours of my report, having made enquiries at the school which obviously backed up my concerns. I have a lot of credibility at the school as I help out a lot, and the SLT know me well enough to know that I make good decisions and am not malicious.
They don’t have ongoing SS involvement, but it was the kick up the backside they needed. The child is so much happier and appears to be thriving, she’s at school on time every day, being fed better, so much more confident and looks happy every day.
Also, I could leave the situation and be confident someone was looking out for her, be it SS or the school.