That sounds like an anxiety dream. Sure things like this might happen. Every once in a while you hear the horror story that is the exception to the rule (the one nanny or parent w/ severe mental illness who kills a child, versus the millions who provide safe loving care all day, every day long).
But you have a nanny you trust who has never given you reason to doubt her. Trust that, and trust your instincts (when you're awake) more than dreams.
Is your son very young, and is he your first? I had my first kids a couple of years ago and I've been stunned by how much of a worrier it's made me now. I think it's a maternal thing, a new parent thing, and a result of watching too much news.
BREATHE, trust your nanny, trust yourself in hiring her, and take a day off to just hang out with your nanny and son and be relaxed if that would help.