use wp_get_recent_posts to get latest posts from specific categories

Hi

I’m using wp_get_recent_posts to get latest posts, but now I need to filter them out from specific categories

Right now I have

$args = array( 'numberposts' => 15 );
$recent_posts = wp_get_recent_posts( $args );

I tried

$args = array( 'numberposts' => '15' , 'category' => '134' );
$recent_posts = wp_get_recent_posts( $args );

Where 134 is the ID of the category I want to get posts, but nothing shows up…

I have 2 main categories and on each one several child categories.

I need to get the latest posts from all the child categories from one of the main categories, how can I accomplish that?

cheers