Override post-formats in child theme

It’s import to set the priority for your after_setup_theme higher than your parent theme. The default priority is 10. Using the twentysixteen as example, use priority 11 on child themes’ 'after_setup_theme' action. Exemple below:

function twentysixteen_child_setup() {
    add_theme_support( 'post-formats', array(
        'video',
    ) );
}
add_action( 'after_setup_theme', 'twentysixteen_child_setup', 11 );