pewc_child_quantity.php

<?php
/**
 * Specify the default quantities here
 * You'll need to update the child product IDs to match yours
 */
function florian_child_quantity( $quantity_field_value, $child_product_id, $item ) {
  if( $child_product_id == 267 ) {
	$quantity_field_value = 3;
  }
  if( $child_product_id == 269 ) {
	$quantity_field_value = 2;
  }
  if( $child_product_id == 271 ) {
	$quantity_field_value = 5;
  }
  return $quantity_field_value;
}
add_filter( 'pewc_child_quantity', 'florian_child_quantity', 10, 3 );

/**
 * Ensure that your child products are pre-selected if the default quantity is greater than 0
 * You'll need to update the child product IDs here as well
 */
function florian_default_product_column_value_before_checked( $value, $id, $item, $child_product_id ) {
  if( in_array($child_product_id, array( 267, 269, 271 ) ) ) {
	$value = $child_product_id;
  }
  return $value;
}
add_filter( 'pewc_default_product_column_value_before_checked', 'florian_default_product_column_value_before_checked', 10, 4 );