Birthday parties are usually 2-3 hours. It is really hard to keep children entertained well at longer events. Usually guests arrive for the first 10-15 minutes. It is great to have a welcoming activity going on like bubbles, a craft that is easy to join .Then the structured party activities, entertainer, games etc Follow this with cake and snacks then gifts. Some large parties open gifts later. Then time to go and give out party favours or loot bags.