Where is the sense behind all the decline comments!
Seriously people, its a charitable event, asking someone to give it a go, free of charge. Its not as if cancer research is asking for promotional shots for their next campaign!
No matter what you ney sayers say something tells me she's going to shoot it anyway.
So! With your equipment OP, you should be able to push the iso up to compensate for low light. Shoot it aperture priority and let the camera work out the shutter speed, only use the flash for say, close quarters shots but try and balance the flash with the ambient light.
Here's a thought, do you have a tripod? I can see potential for a few lovely long exposures, will there be a procession? Can you get a high vantage point? Perhaps a long exposure of people walking with candles making a river of light?
A long exposure of perhaps the people holding candles, you will get movement and the image will be abstract but that might be all you need to capture the ambience.
When I say long exposure i don't mean minutes, more seconds.
Low iso, small aperture (big number like f22) and see what shutter speed it throws back, longer the better.
And if it all goes wobbly then at least you tried and you were there.
How will anyone learn through declining or dropping out?