YADNBU When DS! was a baby, he slept in a cot in our room and when it was a warm summer night like tonight, I'd leave the window open.
However, after Madeline (spelling?) McCann disappeared in whatever way it happened (don't want to start a discussion on it). I said to Dh that we couldn't hear what was going on upstairs at the back of the house where our room is, as were downstairs with the telly on.
I now have 2 DS, age 2 and 1/2, and 4 and I NEVER leave their windows open at night. Too bad if it gets a bit stuffy. I used to leave the window open at the top of the stairs but it's right outside their rooms, so I don't anymore.
I open their windows during the day to air their rooms. Yes, ideally, I'd have some fresh air at night, but their safety and my peace of mind is more important.
I recently got new window locks put on all the windows and the one at the top of the stairs has some kind of thing to limit how far it will open. I'm a fling the windows open wide all day long kind of person but I worry too much about them clambering up and falling out of a 2nd storey window.
DS1 fell out out of a window in a holiday lodge on our first day there, he was "trying to see the birdies". He broke his leg, that was last year and he still remembers. DH was with them and stepped out of the room for a second or 2.
So, no, YADNBU again. Do what makes YOU feel safe and never mind what anyone else thinks!