When a child theme overrides events.css, The Events Calendar looks for the stylesheet in the parent theme.
Could you possibly change the "loadDomainStylesScripts" function in the-events-calendar.class.php to look something like:
public function loadDomainStylesScripts() {
load_plugin_textdomain( $this->pluginDomain, false, basename(dirname(__FILE__)) . '/lang/');
$eventsURL = trailingslashit( WP_PLUGIN_URL ) . trailingslashit( plugin_basename( dirname( __FILE__ ) ) ) . 'resources/';
wp_enqueue_script('sp-events-calendar-script', $eventsURL.'events.js', array('jquery') );
if( $filename = locate_template( array('events/events.css') ) ) {
$templateArray = explode( '/', $filename );
$themeName = $templateArray[count($templateArray)-3];
wp_enqueue_style('sp-events-calendar-style', WP_CONTENT_URL.'/themes/'.$themeName.'/events/events.css', array(), The_Events_Calendar::VERSION, 'screen' );
} else wp_enqueue_style('sp-events-calendar-style', $eventsURL.'events.css', array(), The_Events_Calendar::VERSION, 'screen' );
}