Magento Script To Activate Products of Child Categories in the Parent Category

update_product_categories.php

<?php

error_reporting(E_ALL);
ini_set('display_errors', '1');

// only allow php CLI execution
if (php_sapi_name() !== 'cli') {
    exit;
}

require_once 'app/Mage.php';
Mage::app()->setCurrentStore(Mage_Core_Model_App::ADMIN_STORE_ID);
set_time_limit(0);

// parent categories on the left, child categories from which products should be taken on the right
$categoryMapping = array(
    1 => array(2, 3, 4)
);

foreach ($categoryMapping as $destCategoryId => $sourceCategoryIds) {
    $destCategory = Mage::getModel('catalog/category')->setStoreId(Mage_Core_Model_App::ADMIN_STORE_ID)->load($destCategoryId);
    $postedProducts = $destCategory->getProductsPosition();
    foreach ($sourceCategoryIds as $sourceCategoryId) {
        $sourceCategory = Mage::getModel('catalog/category')->setStoreId(Mage_Core_Model_App::ADMIN_STORE_ID)->load($sourceCategoryId);
        $productsToCopy = $sourceCategory->getProductsPosition();

        foreach ($productsToCopy as $id => $position) {
            $postedProducts[$id] = $position;
        }
    }
    $destCategory->setPostedProducts($postedProducts);
    $destCategory->save();
}

A candidate for your first n98-magerun module with a "category:child:activate-products" command?