I think it really depends on the camper. Some kids just don't do well away from home before a certain age. My brother came to my resident camp for a week when he was 9 and cried pretty much non-stop until my mom came to get him. He was just not good to be away from home overnight like that.
I think on the whole overnight camp is a place for bigger growth. It gives kids time to separate from their parents and start to figure out who they are and what they like for themselves. But for kids like my brother (who did very well in day camp) only being away half the day may be all they need.
