I'm not a parent but I am surprised at the number of posters who felt if the pool danger was removed (with a locked gate or anything else) that it would be okay to leave a child alone in a house. What if an electrical fire broke out? It does happen, and without an adult there ... well, *shudder* Or anything else. And haven't kids gotten into places and through doors and gates that no one thought they could? Five hundred yards away is plenty of space and time for a child to get into serious trouble.
If I were the homeowner they'd never be invited again. And if I were one of the group I'd have been out of there the first day because I feel that wineries and breweries are adult pleasures. You don't go to them to see children. You go to sample and enjoy adult beverages in an adult atmosphere.
YMMV of course.
I think it's fair to say that anything could happen at any time. Being withing 500 yards of a child at all times is pretty much impossible (I know, I've tried!). I agree generally it is not wise to leave a toddler at home alone, but I will not judge a parent for leaving a child in a house and going outside. There are several homes that I've been to that I am comfortable leaving my child asleep in while I go outside in the yard. And this is because I know that I can ensure that she is about as safe as she would be if I were just in a different room of the house. I also think that level of comfort a parent may have will vary by parent, by house, and by child. If my child were a sleepwalker for instance, I would worry a lot more than if I had a 9 month old that sleeps soundly and can't get out of a pack and play.
In this case, the house was not safeguarded enough for the child to be left unattended, so I don't blame the OP for being upset finding a child asleep alone in a room with the pool door open. She is well within her rights to say something to the parents about this. I do not think, however, it would be ok to tell a parent that because something could
happen, they should be closer to their children if there isn't a specific safety issue to point out (I hope that makes sense).