M.S.
At 4-5 years old I would definately have planned activities for them or it could erupt into chaos pretty quickly. Making their own jerseys sounds like a great idea. I usually put crayons and coloring pages on the table for children to do as they arrive. Keeping in mind that their attention span wont be very long I would probably try to plan enough activities to be able to change every 10-15 minutes, if they stick with something longer than that GREAT!! We also put 2 chairs together for gift opening and take a photo of the gifter with the birthday child and their gift, makes a great reminder for thank you notes. Hope your son has a great birthday!!