Complaining at a party is an instant fun killer. Young birthday attendees may get upset for any number of reasons, such as jealousy over a present, food selection or a sugar crash. Along with following the rules, guests must also understand that complaining isn't allowed. For example, boys and girls must be ready to either eat the snacks and cake that are offered to them or know how to politely decline. Explain that the party is being thrown for their friend, and they'll have a chance for their own celebration when their birthday rolls around.
Likewise, the birthday kids should also keep complaining to a minimum. A simple way to prevent this is to include them in the planning process. Once they get old enough, boys and girls can help select their own party theme, activities, guest list and so on. If they express discontent that something isn't going their way during the party, urge them to keep a positive attitude.