Sorry to hear this. I also have two granddaughters and would hate if something happens to them. At the same time, I don't see a point in blaming yourself.

First, it was not your fault.
Second, by blaming yourself you are not helping your granddaughter.

I obviously don't know all the details and it isn't my business. But if I were in your shoes I would try to do what I can to help your granddaughter if it is at all possible in your situation.

If there was abuse I'd contact the police, but make sure you have proof. Just stating that your granddaughter deviated her glance from your scrutiny isn't enough.

STOP BLAMING YOURSELF! You need to have a clear mind so you can DO something!