I don't doubt your words a bit & wish I knew the best direction forward. It might be worth the expense of an autopsy or vet visit to get some medical explanation for yourselves(i have no idea if that's doable-- maybe try a university vet? or ag vet?).
Goin out on a limb here...
The kid will need to gain understanding of the why & how to move forward & heal & put it in perspective. To understand that nobody did anything wrong & that these kinds of things are rare but happen... Maybe a kind veterinarian can help you with this explanation. If there's no solid "why" it could leave a person stuck in nervousness about "what might be" or that we have no control over our lives.
That has to be counterbalanced with all of that love & good; No doubt that was a best friend and a source of security for her & all of you.
In the short term, busy work, chores, distractions can help pass difficult time.
When I was little(about age 3-5) our family dogs were a Great Dane & a miniature Schnauzer. When my neurotic parents were bickering or shouting, you can bet I was hanging out with the reasonable & stable dogs.
I hope this helps in some way.