function.php

<?php
//Get current category ID

$categories = get_the_category();
$category_id = $categories[0]->cat_ID;

// OU

// Get children ID of current category

$args = array(
    'parent' => get_queried_object_id(),
); 

$terms = get_terms( 'category', $args );

$term_ids = wp_list_pluck( $terms, 'term_id' );