$maincategory = Mage::getModel('catalog/category')->load(194); //parent category id
$subcategories = $maincategory->getChildrenCategories();
foreach($subcategories as $category){
$collection = Mage::getModel('catalog/category')->load($category->getId())->getProductCollection();

$subcategory = Mage::getModel('catalog/category')->load(191)->getProductCollection();
    return $subcategory;