Get Children Configurable Products

getChildrenConfProducts.php

<?php

$product = Mage::getModel('catalog/product')
               ->load(11925);

$children = $product->getTypeInstance()->getUsedProducts($product);
$children_ids = [];
foreach ($children as $child){
    $children_ids[] = $child->getID();
}

$childrenCollection = Mage::getModel('catalog/product')
                          ->getCollection()
                          ->addAttributeToFilter('entity_id', array('in' => $children_ids))
                          ->addAttributeToSelect('*')
                          ->joinField(
                              'qty',
                              'cataloginventory/stock_item',
                              'qty',
                              'product_id=entity_id',
                              '{{table}}.stock_id=1',
                              'left'
                          )
                          ->load();

$total_qty = 0;
foreach ( $childrenCollection as $product) {
    $total_qty += $product->getQty();
}

echo "Total QTY is $total_qty";