My aunt and uncle spent a couple of weeks volunteering in an orphange overseas. They went through a registered charity. They paid their own travel, accomodations and meals. They did tell friends and family what they were doing and encouraged them to make donations either directly to the charity or they would take donations of school supplies directly over in their luggage. I supported that through a cash donatuon to the charity.
I think if your friends were paying their whole way and every dollar raised went directly to the orphanage, I'd be inclined to give something. But, it sounds like they're asking for funding for their trip and that's where I draw my own, personal line.