You can get window latches that restrict how much a window can open, with a key for them when you need to open them fully - for example and emergency.
Just be careful that you don't allow them to open too much - I misjudged that and Ada, my little girl, climbed out of the window on the 12th floor
A kitten can get through a very small gap - think the height from the base of their jaw to the point between their ears - they can totally flatten themselves when there is something interesting that they *must* investigate outside.
Everything ended OK for us, but I wouldn't want another owner to feel as I did watching Ada jump up outside the window to get my attention.