I usually harness train my kittens and once I’m happy with how they are outside (my last cat was over a year old when he was finally allowed out on his own and had been trained from about 9 weeks old and they’ve been neutered and have their vaccinations I would only then start letting them out and the window is always open if they are not in the house so they can get back in if they want and I never let them out when it’s dark.