My DS (6) is too young for me to consider it, but I could see doing it later. However, it would probably need to be a child I knew VERY well, who had already done some "challenging" things with the family. DS's best friend is over here all the time, he's slept over, I've taken him hiking, to basketball games, and to another city for a day long trip by train (we rode up to see a new exhibit at the Baltimore Aquarium). So, I feel like I know him well enough to predict what kind of behavior we'd get on the trip.
If DS really wanted to bring someone else I'd probably plan a number of events with the child first to test the waters.