"These things happen all the time these days"
Really? Well, that's bound to be reassuring to Stella, isn't it?
Anyway, Stella, you'd better prepare yourself for a long haul with this, it will undoubtedly take time so you'd better get used to it. For the very simple reason that young children are involved, i.e. your two young daughters, one of whom is the victim here.
You'd want the Police to get it right? Obviously.
Something else puzzling me. You've said this lad's your nephew, therefore I take it he's your sister's or brother's son?
But you haven't made any mention of his parents and I find that a bit odd. I'm sure they're devastated? It's bound to have a wide ranging effect on your wider family? What are they saying about it?
Anyway, if you want to see this matter through you've got to adopt a more positive attitude for your little daughter's sake, and for your eight year old who you say witnessed it.
You are not being treated unfairly, it's the way things are with investigations, there's a lot of work for the Police to do and it's not a given automatic that anyone will get convicted of anything. For I'm sure your nephew will be giving the Police a different version of events than that of your daughters.
I'm sure as well that you're now receiving support from the various agencies who are bound to be involved.