Sub Plugin deactivation when Parent Plugin deactivate

You can use detect_plugin_deactivation,

function detect_plugin_deactivation(  $plugin, $network_activation ) {
    if ($plugin=="parentplugin/parentplugin.php")
    {
        deactivate_plugins(plugin_basename(__FILE__));
    }
}
add_action( 'deactivated_plugin', 'detect_plugin_deactivation', 10, 2 );

Add this snippet in your plugin’s main file.