Probably Not Too Scary for Kids But Poor Kitty
I hadn't seen The Fly--the original from 1958--since I was a young child over 50 years ago. I have never seen the 1986 remake nor do I ever intend to. The original film is termed Comedy/Horror on Amazon Prime Video, but rated 18+ for frightening scenes and smoking, perhaps a bit extreme as I was about ten years younger than 18 when I first saw this and I turned out all right.
Amazon's print must be a restored version as it is absolutely beautiful--the color, picture, and sound are perfect. Previously I had only seen it on a small, black-and-white TV set and admittedly some scenes near the beginning and end are terribly gory on a good sized screen with a sharp clear color picture. A few frightening scenes appear throughout but it's mostly not that scary or gory. Huge trigger warning for animal lovers: in one scene a scientist working from home loses the beloved family cat in an experiment. I was yelling, "NOOOOOOO!!! NOT THE KITTY!!!!!" He doesn't admit this to his wife right away and when he finally does, she just says, "How could you?" and moments later is laughing and drinking champagne with him. Her reaction to the loss of the cat is just as troubling as the loss itself.
The versatile Vincent Price is a sympathetic good guy here, but...not only does he smoke, he offers alcohol to a minor, and I am talking extremely minor, like seven years old. They are portrayed as being part of the French culture, in which it was normal until fairly recently for children to have watered-down wine.
Near the end of the film you will experience one of the most upsetting scenes in cinema history, with a trapped victim repeatedly frantically screaming for help. This scene is so legendary the screams will never leave your head, and the visual is also surreal and disturbing in the extreme. I wouldn't advise to keep kids away entirely but you might employ some caution for children under ten. Personally I remember enjoying the film as a child.