The smallest sleeper suit size seems to be Small. They don't always have them, and they run a little full and long for us petite adults, never mind a child, so just roll up the sleeves and trousers legs and pull the string on the waistband tight.

As to food, it depends on the child. The menu in F has a wide variety of options, and some would appeal to some children, surely.

IFE has lots of child-friendly options.

You're a great dad for doing this. Some of my best memories are of flying with my dad to Europe in Pan Am and TWA, in F (which was not nearly as fancy back then). I felt like a big girl and behaved accordingly.