If you find yourself saying, "No matter how hard I try and try, I can't make my kid do X ..." or "No matter how hard I try, I can't make my kid understand Y ..." it's usually a clear sign that expectation and enforcing that expectation are a significant part of the problem. Your expectation may in fact accurately address the mean—that is, you may expect a behavior of your 9-year-old that most 9-year-olds can do—but remember the range of human variability and try to structure antecedents (the things you do to encourage a behavior to occur) with room for that variability.
This is of course harder to do when you're surrounded by parents whose kids nicely hit or exceed that behavioral mean.
The author of this piece, child psychiatrist Alan Kazdin isn't telling you to eliminate expectations for your kids, but you should keep in mind that the time it takes to reach developmental milestones really is variable, and what comes easy to your neighbor's nine-year-old may not come easy to yours, even if it is something as basic as not breaking down and crying like a 3-year-old in the grocery store.
Biology is all about variation, and while we need to worry about pathological variation, we shouldn't drive ourselves insane more benign (but frustrating) forms. The good news is (as Kazdin points out) that at lot of this childhood behavioral variation doesn't correlate strongly with what kind of adult your kid will be.
