pewc_child_product_independent_quantity.php

<?php
/**
 * Force a child product to be automatically selected
 */
function prefix_filter_default_child_independent_quantity( $quantity_field_value, $child_product_id, $item ) {
  if( $child_product_id == 9999 ) { // Change this to your child product ID
    return 1;
  }
  return $quantity_field_value;
}
add_filter( 'pewc_child_product_independent_quantity', 'prefix_filter_default_child_independent_quantity', 10, 3 );

/**
 * Set the default quantity when the user toggles on and off the checkbox
 */
function prefix_set_child_quantity_default( $set, $post_id ) {
	return 'yes';
}
add_filter( 'pewc_set_child_quantity_default', 'prefix_set_child_quantity_default', 10, 2 );