functions.php

<?php

add_filter('avada_metabox_tabs', 'avada_child_metabox_tabs', 10, 2);
function avada_child_metabox_tabs( $tabs, $post_type ){
	$screen = get_current_screen();


	if( 'avada_portfolio' == $screen->post_type ) {

		$tab_key  = 'avada_child_portfolio';
		$tab_name = esc_html__('Child Portfolio', 'textdomain');

		$tabs['requested_tabs'][]     = $tab_key;
		$tabs['tabs_names'][$tab_key] = $tab_name;
		$tabs['tabs_path'][$tab_key]  = get_stylesheet_directory() .'/tabs/tab_'. $tab_key .'.php';

	}


	return $tabs;
}