The childrenCount() function returns a count of how many child pages belong to the current page. A simple echo $this->childrenCount(); will return the number of “published” pages to the current page.¹⁾

childrenCount() can be useful, then, for determining when to include navigation, for example:

<?php if ($this->childrenCount() > 0) {
  // if count is > 0, there are child pages, so do stuff
  } else {
  // no child pages, so do something else
  }
?>

It takes the same parameters as the children() function; see the documentation there for details.