[Badger]

OP is neither a travel agent, nor these people's mother. I think if you have a plane ticket somewhere you should know what time it is. OP did screw up however, and if I did something like this I would feel bad and offer something to my friends based on the cost of rebooking.
The financial standing of the people involved shouldn't be a factor, but I'd probably be inclined to chip in more if I was much better off than my friends.

EDIT: So I didn't really weigh in on OP's question. If the cost was over $100 each I'd probably chip in $100-$150 over my cost. If it was less than $50 each I'd probably offer to take them out to dinner or something.