MrsTerryPratchett: "In a situation where a girl had drunk bleach, it could be depression, it could be another MH issue, it could be that she is scared to death, it could be her family, it could be a boyfriend, it could be an accident, it could be something else. You have to find out and if you go in all guns blazing, you never know until it is too late. You need someone to trust you and talk to you. SWs aren't psychic."
I have some sympathy with the difficulty, and I couldn't do your job. Nonetheless, if a child has drunk bleach, then in a way it doesn't really matter whether it's a mental health issue or whether it's because she's scared to death, does it? You would want to keep a very close eye on that situation. I know someone who drank bleach accidentally and the damage was horrific and long-lasting. With Shafilia, other warning signs were there.
It reminds me of several other cases (Victoria Climbie, Baby P etc) where there just wasn't any joined up activity going on with the agencies. If you looked at all the evidence that different agencies have (ie social services, the school, the police) you can build up a compelling case that someone is being abused much more easily than if all these agencies act alone.