Another suggestion:
I have modified the search.php file in modern theme.
<?php if ( osc_count_categories() ) { ?>
<div class="row checkboxes">
<h6><?php _e('Category', 'modern') ; ?></h6>
<ul>
<?php // RESET CATEGORIES IF WE USED THEN IN THE HEADER ?>
<?php osc_goto_first_category() ; ?>
<?php while(osc_has_categories()) { ?>
<li>
<input type="checkbox" name="sCategory[]" id="sCategory" value="<?php echo osc_category_id(); ?>" <?php echo ( (in_array(osc_category_id(), osc_search_category()) || in_array(osc_category_slug()."/", osc_search_category()) || count(osc_search_category())==0 ) ? 'checked' : '') ; ?> /><label for="cat<?php echo osc_category_id(); ?>"><strong><?php echo osc_category_name(); ?></strong></label>
<?php // MOSTRAR SUBCATEGORIAS ?>
<?php if ( osc_count_subcategories() ) { ?>
<ul>
<?php while ( osc_has_subcategories() ) { ?>
<li style="padding-left:20px;">
<input type="checkbox" name="sCategory[]" id="sCategory" value="<?php echo osc_category_id(); ?>" <?php echo ( (in_array(osc_category_id(), osc_search_category()) || in_array(osc_category_slug()."/", osc_search_category()) || count(osc_search_category())==0 ) ? 'checked' : '') ; ?> /><label for="cat<?php echo osc_category_id(); ?>"><strong><?php echo osc_category_name(); ?></strong></label>
</li>
<?php } ?>
</ul>
<?php } ?>
</li>
<?php } ?>
</ul>
</div>
<?php } ?>
With this modification, in the category selector now appear the subcategories.