Make sure children understand they must stick with airline staff. Sometimes young children get off with a well-meaning family who has offered to walk them to their gate. Or veteran preteen fliers decide to test their independence by pulling off their tags and wandering over to a gift shop. Even a brief separation puts the child at risk, terrifies the staff and disrupts airline business until the child is found. While ticket-shopping, ask about other amenities for kids.
Most airlines offer special meals catering to kids' tastes.
When the child is on the plane, boredom may lead to loneliness and fear. If there are siblings or other children on the flight, that may help. I know this sounds a little Martha Stewart, but wrap the goodies in tissue paper or the funny papers. If the flight is long, consider packing a lunch. The adult at the other end of the flight plays a crucial role.