Dear S.,
First off, I think that it is a great idea and I have quite a few friends who did do it with their kids and it was the parents idea, not the kids.I think we live in a society that is very self centered and its ideas like this that help kids (and adults) think about those who aren't as materially blessed as we are and also become a little more other focused. IT would also be a GREAT thing for your son's friends to learn from too.
Here's what I would do -
I think the first is to discuss it wiht your son. Talk to them about how you think it would be a cool idea to maybe give kids who don't have toys (elaborate on that) some new things. I would still tell him that he'll still be getting gifts from you and your husband and family. I would also encourage you and your husband to do the same, like a cool family tradition.
Then I would brainstorm with him different charities that are out there that help kids (homeless shelters, Ronald MCDonald house by Children's, Children's hospital, Crisis pregnancy centers) and then let him pick where he would like to donate stuff to.
Then I would visit that place, talking to the organization before hand. I know if you go to the ronald McDonald house or a crisis pregnancy center or William Beaumont Royal Oak, they would be very happy to encourge your son, show him around and really empathize what a help his donated gifts will be. I would include on the invites what he wants his friends to bring to donate. Take your cues from him after he visits.
Maybe even let him take pictures at the place so he can show his friends where the gifts would be going. At 8 he would probably be happy to be able to take pictures and be "in charge" of knowing what kind of supplies they need.
I would also have the gifts come unwrapped to the party (that's what I've seen anytime I've done or attended a party like this) and then let your son go with you to drop it off. I'm sure you will all walk away with a good feeling from helping others and being able to make a difference in the community. I know that almost all of the parties I've gone to wtih kids who have donated their gifts, the kids have beamed iwth joy and pride when dropping off their gifts and have willingly and dillgently wanted to do the same the following year. All my friends who have done this always take their kids out for ice cream or something after dropping them off and make sure to really affirm how proud they are of their child for helping others, etc.
I would also make sure to bring the addresses and names of the kids (tell the organization NOT to put the names on ANY mailing list) and ask the organization to send out "Thank you" to the kids. Kids love getting mail esp. from someplace "official"
I have done this before for my own birthday party a few years back and it was a huge hit. There's lots of online wording available for donating gifts. I also have been to quite a few parties where the parents have done just what I suggested. I would like to say that the kids have not been traumatized at all nor will they grow up and be on Dr. Phil explaining how their entire life was ruined because they didn't get 20 gifts on THEIR birthday. They didn't cry for long durations and they certainly weren't lacking in gifts (they still got family ones) but they did gain a deeper apprecaition for all that they have, learn about charities in their area, see the value and blessing in helping others and got that wonderful "feel good" feeling from helping those around them.
I applaud you for thinking about doing such a thing and hope that you go for it, if you still feel that's in the best interest of your family. If you do, PM me. I would like to send your son a bday card thanking him for caring for those around him = : )