Be frank about money matters with the parents; make sure it's clear who pays what before the friend is involved. If you need financial assistance from the other family, make that evident in the first discussion: ("We can pay for transportation and hotel, but if you could chip in for food and souvenirs, that'd be great.") If they offer to help and you accept, determine amount, designated purpose and timeframe. Is it for tickets or mementos? Is it a pre-trip payment or a reimbursement? Is it a fixed amount or within a stated range?
My husband and I believed that, since my niece's company would enhance everyone's trip, we'd cover transportation, food and lodging. She'd need to provide souvenir money, and she was mature enough to handle her own cash. If you take a friend who has spending money, clarify with the parents who's going to hold onto it and how it's going to be doled out.
Selection, permission, expenses -- those are the essentials, but what else do you need to consider before hitting the road?