I don't know about where you're living but over here the only time I've heard "kids should be kids" was to a parent who had their kids scheduled to within scant nanometres of their lives.

_{I had talked to the parent who got told this and they were proudly telling me about how their kids' pretty much every waking moment was at school or an activity or travelling between those and how their day started at 5am or something like that because some of the activities were before school which wouldn't have been a problem if that was what the kid wanted but I couldn't say that confidently}

Could just be one of those socioeconomic things though :)