I never liked the idea of handing out 'goodie' bags to kids leaving a party. Kids receive way too much junk and trinkets nowadays, and it mostly winds up in the landfill. Besides, aren't they going to be eating stuff like pizza, cake and ice cream there?- why take home candy for later consumption as well?

I think the idea about giving things for the humane society is a wonderful one, and teaches more about the joy of giving than about the joy of getting. What a great example to set!
Does the humane society have a website with photos of some of the current animals?- if so you could print out some of the pictures, with the animals names on them, so the kids can see 'who' is getting their donated items.
Take a picture of the group of kids standing holding their donations, and print it out for the kids to take home to show their parents, and give a copy to the humane society when you bring the donations. Also consider bringing a copy to the local newspaper- maybe they will run a little blurb about what the kids did, and it might set off an example that other follow! The birthday girl would be much admired for her inspired idea then.

Far better than 'goody bags' is to just have some interesting activities for them to enjoy at the party, in my humble opinion. Maybe they could make little catnip mice or pillows, or dog tug toys out of braided fabric (for the animal shelter) at some point during the party, and add them to the animal shelter donation- just one thought.

Do 'all' 8-12 yr old kids actually have computers, ipods and flashdrives these days? Yikes.
If there was one kid there who didn't, they might feel bad getting something like a flashdrive or itunes coupon.