It's widely held false belief that children are good at learning second languages.
I am not sure it's false, that may depend on how do you define "children".
I personally know a couple of kids who were forced to learn a different language around the ages of 4-5. The process went much faster and easier than with adults.
To expand on Douglas_Knight's answer, if it seems counter intuitive that is because children spend basically all their waking hours in a learning environment for nearly two decades of their life. Most of that time is spent learning or using in some way one or more languages.
The typical adult method for learning a second langauge, on the other hand, is to spend an hour or two a week in a classroom or with a tutor. No wonder it doesn't work as well.
To compare apples-to-apples, consider for example the Monterey Naval Postgraduate School which trains American ...
NPR reports on a study giving volprioc acid to adults and training them on pitch (singing):
Brain plasticity is useful for a whole lot more than learning pitch. As the article notes it would be invaluable for training one's ear to pick up sounds of foreign languages, but also it seems reasonable to this commentator that high levels of plasticity during rationality training or other forms of self-development would result in more transformative results.