I'm so sorry for your loss, of your chicks and your coop. At least most of the flock did survive, relatively unscathed. I'd use neosporin (w/o pain medication) or blue kote on their burns to prevent infection and/or pecking by flockmates. As for pain, unless their exhibiting distress, I wouldn't worry too much. Chickens are so dang tough and resilient, and heal so quickly, it's amazing. So if you can keep the wounds clean and keep other chickens from picking at the wounds, they'll be healed up pretty quickly.
As for the cause of the fire, it could be lightning. Even with all of the trees around, lightning will take the path of least resistance, which could be just about anything at any time and might have been your metal fence, which would have inevitably arced to the coop.
However, if it was lightning, I'm surprised so many of you chickens survived in such good shape and weren't stunned or outright killed if their were a striking bolt. If you can (and haven't disturbed too much), I'd call your local Fire Marshal and see if they'd inspect it and possibly be able to determine the source of the fire; better safe than sorry.
As for the cause of the fire, it could be lightning. Even with all of the trees around, lightning will take the path of least resistance, which could be just about anything at any time and might have been your metal fence, which would have inevitably arced to the coop.
