$_category  = $this->getCurrentCategory();
$_children  = $_category->getChildrenCategories();
foreach( $_children as $child ){
    if( $child->getIsActive() ){
        echo "<p>" . $child->getImageUrl() . "</p>";

bool(false)

public function getImageUrl(){
    $url = false;
    $image = $this->getImage();
    if ($image) {

/* @var $_children MagentoCatalogModelResourceModelCategoryCollection */
$_children = $this->getCurrentCategory()->getCollection();
$_children->addAttributeToSelect('image')
    ->addAttributeToFilter('is_active', 1))
    ->addIdFilter($_category->getChildren())
    ->setOrder('position', MagentoFrameworkDBSelect::SQL_ASC);

foreach( $_children as $child ){
        echo "<p>" . $child->getImage(); . "</p>";