widget-mailchimp.php

Widgets do not work this way. Only template files work that way.
You could remove the default widget and add add your code for the custom widget in sweetdate-child/functions.php. You can remove the mailchimp widget like this:

 function unregister_default_widgets() {
     unregister_widget('SQueen_Mailchimp_Widget');
 }
 add_action('widgets_init', 'unregister_default_widgets', 11);