pewc_coupon_is_valid_for_product.php

<?php
/**
 * Exclude child products from coupons
 */
add_filter( 'woocommerce_coupon_is_valid_for_product', function( $valid, $product, $coupon, $values ) {
    if ( ! empty( $values['product_extras']['price_with_extras_discounted'] ) ) {
        $price = $values['product_extras']['original_price'];
        $price_discounted = $values['product_extras']['price_with_extras_discounted'];
        if ( $price != $price_discounted ) {
            $valid = false;
        }
    }
    return $valid;
}, 9999, 4 );