Adults constantly raise the bar on smart children, precisely because they're able to handle it. The children get overwhelmed by the tasks in front of them and gradually lose the sort of openness and sense of accomplishment they innately have. When they're treated like that, children start to crawl inside a shell and keep everything inside. It takes a lot of time and effort to get them to open up again. Kids' hearts are malleable, but once they gel it's hard to get them back the way they were.