子菜单页面未显示在管理仪表板上

I created a plugin which creates a menu page that contain 2 sub menus:

function create_menu() {
    add_menu_page('Main Page', 'Main Page', 'manage_options', 'main-page', 'main_page_function');

    add_submenu_page('main-page', 'First Child', 'First Child', 'manage_options', 'first-page', 'first_child_function');

    add_submenu_page('main-page', 'Second Child', 'Second Child', 'manage_options', 'second-page', 'second_child_function');
}

// Actions
add_action('admin_menu', 'create_menu');

On the dashboard I see the main page name "Main Page", When I hover over it or click on it, the other 2 sub menus are not shown.

Why is that?