// ** we need to include the file below because the activate_plugin() function isn't normally defined in the front-end
include_once( ABSPATH . 'wp-admin/includes/plugin.php' );
// activate pre-bundled custom plugins
activate_plugin( 'plugin-folder-name/plugin-name.php' ); // must have this plugin installed on child theme demo
activate_plugin( 'widgets/my-widget.php' );