With young children, unless the child is hyper, they often go to sleep during the flight. Avoid giving them drinks, food, sweets that might make them hyper. My grandson recently flew for the first time. Admitedly he was a few months past his 3rd birthday - and it was only for a couple of hours but he was as good as gold. As mentioned above, lots for him to see and learn about. He was fast asleep for his first landing though. Slept for most of the return too.