Lol chicken soup was the first thing I thought of when I saw the thread title! I remember my mum used to give us chicken broth and vegemite on toast, I guess it depends on what they are sick with?
Soup is always great though, as it's warming and filling at the same time but not too difficult to eat, especially if you don't feel like eating anything.
Or even some jelly & custard for something a bit special.