They can give you boiled water (hot) in a container and I have also been given it in an air sickness bag. . You then sit the bottle or baggie or jar of food in it until you think it is warm enough- be sure to shake and stir.
My personal best tips are Breast or bottle feed on take off and landing to keep baby's ears from hurting. If it is a long haul then see your DR to see if he advises "sleepy medicine" lol.