In my case, the problem wasn't with my friend, it was with her parents, and it took years for the other shoe to drop. My family and I took our second trip to WDW when I was fifteen. Since there are three children in our family, my parents decided we needed to bring one friend to round out our group, and as the youngest, it fell to me to pick the right friend. We decided on my friend, Jenny, who I had known since I was in first grade and is still my best friend to this day. It was Jenny's first trip and my parents made all of the arrangements with her parents. Her family paid for her admission and gave my mother her food and souvenir money, which was doled out on a daily basis. We all travelled to the parks together each day and made arrangements to meet at a specific time and place. We were usually on our own for no more than a couple of hours each day, and we knew better than to be late, otherwise we'd lose that privelege. All in all, it was an excellent vacation. Jenny and I had been drifting apart a little during junior high and freshman year, and the trip brought us closer together.
Unfortunately, I went on a week long camping trip with Jenny's family two years later and found out the hard way that her parents kind of resented the fact that we took her to WDW, when they couldn't (or wouldn't). That fight took a few months to sort out. I think it's equally important to consider the child's parents as it is to consider your young guest, herself.