Me personally, I would spend a few bucks to get a good setup. I think if you did, you might actually be able to raise quite a bit more money. You are picking a great time to start thinking about this as well as you have a few months to shop around. There are always Xmas backdrops for sale on Ebay, so you can pick and choose the one you want and hopefully get it at a good price. Then you need a backdrop stand and a few small lights. I would say $400 or so might be all you need. You already have the camera. I can't see it being more than $500 even with wireless triggers and everything. Really you shouldn't be shooting more than a two or 3 people at a time, so you don't need tons of lighting. It'd be good if you could get something that will work with or without Santa so you aren't limited to shooting only when you have a Santa. But you could probably charge a few more bucks with Santa. He is the big draw after all...
Is it out of the realm of possibility that if you can bring in enough money, you can still make a healthy contribution to your cause while subsidizing some of your equipment costs?
I guess it depends on how serious you want to be with it and how much income potential you're looking at. Judging by your camera, I would venture to say that you wouldn't mind having a light kit at your disposal whenever you want it. I think that if you could produce some good quality Christmas portraits, you will raise some money. If people are counting on you for their seasonal portraits, and not just going to show their support for the cause, you could really increase the size of your contribution.
It could be a great win-win-win situation. You get some equipment at least subsidized, people in the community get some good quality Christmas portraits, and most importantly, the bushfire victims get some much needed help. I commend you on your efforts on the victims behalves. They are lucky to have you. Best of luck to you on this endeavor. Hope this is at least a little helpful. - Bob