If you re-read some of the posts on this board about how the adults grew up, everyone seems to have a "story"--a lot with substantial "challenge" or overcoming difficult childhoods. I always wonder whether the difficulties bring out resourcefulness. We want to pave a sunshine-y path for our kids (isn't that natural?), but did the bumps in our lives actually do us some service? Is that what makes us special? (I'm rambling, too). Though I'm not proud of it now, I remember going big name university thinking I was better than the other kids there because they were all rich and went to fancy prep schools and had stable families, while I had weathered something not so perfect (now I know that every has their issues).
Anyway, I really do think challenge is good, though it doesn't have to be unpleasant. We can raise the bar for our kids (and ourselves, though I haven't done that yet really) by goal-setting including trying new and different things. It can be athletic, a language, musical instrument, entering contests, etc. I try to develop goals with the boys' input, like taking an AP exam (at 13, a year after a summer institute), or the SAT/ACT testing, or taking fencing. I'm sure there are every day challenges of being younger than classmates in high poverty school. I'd like to protect them from traumas that could be considered challenges.