Well, today was my first experiment with having a combined birthday party. Since 2 of my kids have May birthdays and 2 have June birthdays, I told them that they could either each have a big birthday party every four years, (taking turns), or they could have a combined party every year. They opted for the combined party.
It went well!
We had about 20 kids total. The invitations had said that it was for a combined birthday party, and then said, "You will be the personal guest of ..." This way, the parents didn't think they needed to bring 4 presents. During the party, we separated the kids briefly into "mini-parties" for the gift exchange. Worked well. Each birthday kid wound up receiving about 2 gifts. (Most of the kids had siblings who also were invited, and when we divided up, we had whole families as the personal guest of just one kid.) So, that was nice. They still got presents, but it wasn't the "big" thing of the party.
Theme was carnival. All very made-at-home, no fancy clowns or inflatable jumping things. And the kids had a blast! Bowling with empty 2 liters as pins, bean bag toss, rubber duck pond, etc. Prizes for every game, and all who played won a prize. Cheap prizes, even better than Oriental Trading:
Thanks to Sams, we had a nacho stand (nacho sauce in a crockpot), hot dogs and veggie dogs (in hot water, in separate crockpots), and all the condiments. Set up the chocolate fountain, which was definitely the hit of the party. We adults may think they're hokey, but kids LOVE them.
Rented a big (uncarpeted) room at the church, so we didn't have to worry about weather. Side benefit: those families NOT from our church got to see where our church was.