I get what you say about unneeded gifts. The last 3 years, one of my daughters has asked for donations to different charities. We will include a list of ideas that we get from the wish list of the charity she is collecting for. One year she did the Ronald McDonald House. Her party consisted of going to the American Girl store, and then heading over to the RM house and all the party goers got to see where their donations were going as well. The next year, 2 of her friends that went to the party collected donations instead of gifts at their parties as well. How awesome it that! Every now and then, a friend will still bring a gift, but that's okay. And of course she gets gifts from family. (trust me, she's taken care of.) I agree with the other people who said it's hard to come to a party empty handed. This way, they don't have to. It's been a lot of fun picking out the place to donate to, and what she wants to donate. If you want some ideas on this, contact me.
Additionally, I have twin girls. One year they also donated their gifts. What i did, I have their party together, of course. But half the invitations were from twin A and went to her friends, and the other half were from twin B and went to her friends. They did different charities, and so the donations were different as well. Good luck. We have really enjoyed it in our family, and I see more families doing the same, so people are getting use to the charity donations thing. What a loving thing you are doing, and teaching your children.