Get the child category ID of current category

Question

I am trying to get the category ID of the child (no grand children) of the current category. Using below method:

$category = get_category(get_query_var( 'cat' ));
// get category ID
$catid = $category->cat_ID;

I can get the current category ID. However, using this in WP_Query result in displaying the posts that are published in Parent, Child, Grand Child category.

Is there any way, to get the child category ID of the current category?

Update #1:

Sample category structure is as follow:

- Parent Category x
-- Child One
--- Grand Child one
--- Grand Child two
-- Child Two
--- Grand Child one
--- Grand Child two

once in the Parent Category x we have the ID, now I want the Child Category id. Once in Child Category x, I want the Grand Child Category id to be used in :

wordpresss/category/parent       : posts published in child category 
wordpresss/category/parent/child : posts published in grand child category

Answer 1

Use get_terms with parent argument to get only direct children of a term. Use wp_list_pluck to extract an array of term IDs that can be passed to a query.

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

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

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

Also uses get_queried_object_id to get the ID of the current category archive.