prefix_woocommerce_check_cart_items.php

<?php
/**
 * Check if there are any out of stock child products in the cart
 * Remove the parent product if so
 */
function prefix_woocommerce_check_cart_items() {

	// If we've enabled the check

	foreach( WC()->cart->cart_contents as $cart_item_key=>$cart_item ) {

		if( ! empty( $cart_item['product_extras']['products']['child_products'] ) ) {

			// This is a parent product so we might need to check that the child products are still in stock
			foreach( $cart_item['product_extras']['products']['child_products'] as $child_product_id=>$data ) {

				// Iterate through each child product and check its stock level
				if( apply_filters( 'pewc_check_child_product_stock', false, $child_product_id ) ) {

					$child_product = wc_get_product( $child_product_id );

					if( ! $child_product->is_in_stock() ) {
						// Check whether it's still in stock and if we need to prevent purchasing the parent product
						wc_add_notice(
							apply_filters(
								'pewc_child_product_out_of_stock_message',
								__( 'One of the options in this order is out of stock so the product has been removed from the cart', 'pewc' )
							),
							'error'
						);

						// Remove the main product from the cart
						WC()->cart->remove_cart_item( $cart_item_key );

					}

				}

			}

		}

	}

};
add_action( 'woocommerce_check_cart_items', 'prefix_woocommerce_check_cart_items', 10 );

add_filter( 'pewc_check_child_product_stock', '__return_true' );