How to Append ACF Fields to Child Product Categories only?

The first thing I would recommend is reading this article on official ACF website. There is a nice description about all aspects of custom locations.

To create a custom location rule that allows adding ACF fields to the product categories that are children only, just add the following code to your functions.php file:

add_filter('acf/location/rule_types', 'acf_location_rules_types');
function acf_location_rules_types( $choices ) {
  $choices[ 'Post' ][ 'taxonomy_term_child' ] = 'Taxonomy Term Child';
  return $choices;
}

add_filter('acf/location/rule_values/taxonomy_term_child', 'acf_location_rules_values_taxonomy_term_child');
function acf_location_rules_values_taxonomy_term_child( $choices ) {
  if ( $taxonomies = get_taxonomies( array(), 'objects' ) ) {
    foreach( $taxonomies as $taxonomy ) {
      $choices[ $taxonomy->name ] = sprintf( '%s (%s)', $taxonomy->label, $taxonomy->name );
    }
  }

  return $choices;
}

add_filter('acf/location/rule_match/taxonomy_term_child', 'acf_location_rules_match_taxonomy_term_child', 10, 3);
function acf_location_rules_match_taxonomy_term_child( $match, $rule, $options ) {

  // Apply for taxonomies and only to single term edit screen
  if ( ! isset( $options[ 'taxonomy' ] ) || ! isset( $_GET[ 'tag_ID' ] ) ) {
    return $match;
  }

  // Ensure that taxonomy matches the rule
  if ( ( $rule[ 'operator' ] === "==" ) && ( $rule[ 'value' ] !== $options[ 'taxonomy' ] ) ) {
    return $match;
  }
  elseif ( ( $rule[ 'operator' ] === "!=" ) && ( $rule[ 'value' ] === $options[ 'taxonomy' ] ) ) {
    return $match;
  }

  // Get the term and ensure it's valid
  $term = get_term( $_GET[ 'tag_ID' ], $rule[ 'value' ] );
  if ( ! is_a( $term, 'WP_Term' ) ) {
    return $match;
  }

  // Apply for those that have parent only
  if ( $term->parent ) {
    $match = true;
  }
  else {
    $match = false;
  }

  return $match;
}

Afterwards, you’ll have the following option under Location settings of your Field Group: