We have done this combining three girls. My next door neighbor has two daughters, the eldest being my daughter's best friend. When the girls were turning 6 and 9; we had two parties in one using both back yards. The 6 six old had friends over from 4 to 6 pm and the 9 year old friends were invited from 5-7 pm. From 5-6 pm, we had sack races, limbo, duck/duck/goose, dancing to music, and general running around for approximately 8 kids. With parents, grandparents,aunts and uncles, there was abundant supervision for all of the kids. When the older kids began to arrive, we started serving snacks, drinks, and candies. When all kids were present, at around 530, we brought out three cupcakes with candles and all sang happy birthday to the three birthday girls. We then brought out all of the rest of the cupcakes to pass around and presents were opened for all to see. By this time parents were arriving to take their younger kids home, leaving mainly the older kids for the next party. (We did have the 6 year old's best friend stay over so the sis didn't feel alone). Now time for the older kids party with similar games and adding a few more to keep them running around and occupied. Parents picked up all of the older kids by 715-730. Clean up took little time with all of the help involved. The 6 year old and her friend, and the 9 year old and my daughter all spent the night at the neighbor's house. While we sat out on the deck and had some relaxing adult time, the girls watched a DVD and ate a much more nutritious snack. They were all asleep by 10:00.
Cost? Approximately $120 total. We made the cupcakes at home, juice bags, water bottles, snack crackers, cheese cubes, and popcorn. We gave little trinkets in brown lunch bags that the girls had decorated in colorful "thank you's" ahead of time; stickers, little toys, yo-yos, candies, etc. We had most of the games and equipment already except for the pillow cases which we got at a thrift store for a quarter each. We counted heads and had well over 30 kids total at the combined parties.
It takes quite a lot of planning to make sure you have plenty of games to do; we used half of the games we'd planned but we were ready with more. Enlist several FUN adults and have several games going on at once and rotate the kids through them. Enlist the kids to help out, this keeps them so busy, they don't have time to think about who's party it is supposed to be. Stay on time with the activities or you'll run into trouble with the merger of the two groups.
Our kids have such a wonderful time, they ALL still talk about that party.