I have been thinking about "good stress". There is such a thing, and what constitutes "good stress" will depend very much on the individual and his reaction. This applies to children as well. Sometimes called "eustress" to distinguish it from "distress", good stress is a taxing of mind, body or spirit that leads to growth and a sense of self-worth and fulfillment.
So in light of that, how do we challenge our children appropriately and guide them to react to stress in a positive way?
The Greek prefix "eu" means well or good. Stress is a Latin word -- the closest Greek equivalent looks like anagke. Some other related words are Tolkien's eucatastrophe (meaning the happy reversal that comes at the end of a redemptive story) and eudaimonia (Aristotle's word for happiness, which connotes something like a good spirit or good fate).
Charlotte Mason talks about the infant kicking his feet for sheer joy; but at the same time, he is developing the very muscles that will later help him crawl and walk and run.
Sports can be a "good stress" depending on the child. I remember when my younger two boys were in T-ball. I saw one child being literally dragged into the field by his mother. He was screaming up a storm. Of course, no one could force him to actually play, and he ended up at the "bench" (really, for these little ones, a blanket spread upon the grass). I was the supervisor of the bench, to keep the bored youngsters from kicking each other and getting into the sort of mischief that little kids get into when they have nothing to do. I looked into this little boy's eyes and saw that he had shut off. His eyes were like the sky -- empty and bright. I felt as if I'd seen a little tiny creature curled up inside his soul, afraid of everything and unable to uncurl. So the exterior was all unresponsive amusement, protecting the stressed spirit inside. He was hard to supervise. I could totally relate to his young mother who probably thought she was teaching him character by forcing him to play, and perhaps indirectly she was helping his character formation by her intense sincerity. But in the moment, this boy was a distressed creature incapable of learning anything.
This, I would say, is not good stress, and I look for the kind of things that will encourage my children to expand, not contract into a miserable psychic ball. I think this differs for everyone. I know that the level of stress that feels like a positive charge to a choleric or sanguine child was enough to push me to the end of my coping devices when I was a small melancholic child.
Sometimes, though, a child needs a bit of exhorting or a nudge of some sort to reach his positive stretching point. This is probably my weakest point as a parent; I get too easily immersed in the anxiety of the child; it reverberates as my own anxiety. I think I probably arrange life too much to suit my own stress tolerance level. So definitely, there is a balance.