As ken and newjudge have advised - the process - or not - of these cases is decided by the realistic prospect of a conviction which is in the best interests of all involved.
In this, the law is utterly dispassionate, as it has to be, which means that their decision can appear competely heartless and cruel.
Knowing something, even backed up by apparently incontavertable evidence, is not the same as proving it in a court of law, and that is where the CPS comes from.
I know this offers absolutely no sense of justice to you or your daughter, and your family, but this is the system, flawed though it is, that we live with.
I can only hope you all of you can move on from this dreadful incident, and from the tone of your posts, you are obviously a wonderful mum, and will support your child through this horror and out through to the other side.
I am sure right now it feels as though there is no 'other side', and your daughter will never get over this.
It's true, she won't 'get over it', but she will learn over time to assimilate it and weave it into her experience, and eventually, it will cease to dominate her in the way it does now.
That day will come, hang on, and keep coming back here, there is always depthess support waiting for you.