WordPress Events Manager: personalized single category page lost “current-page-item” CSS class

I have a category of events (A), it has 2 sub-categories(A1, A2). I need to list all its events grouped by sub-categories:

Category A 

 Category A1
 Some description text
 Coming events : blabla (these events are sorted by the location)
-------------------------------------------------------------------
 Category A2
 Some description text
 Coming events : blabla

The default single category page does not display its child categories’ events, so I modified the file templates/templates/category-single.php:

global $EM_Category;
$arrChild = get_term_children($EM_Category->id, EM_TAXONOMY_CATEGORY); 
$child_count = count($arrChild);

if($child_count == 0) 
    echo $EM_Category->output_single();
else {
    for($i=0; $i < $child_count; $i++){
        $cat = new EM_Category($arrChild[$i]);
        echo '<h2>'.$cat->name.'</h2>';
        echo $cat->output_single(); 
        if($i < $child_count-1)
            echo '<hr/>';   
    }
}

It displayed the events as I expected.

When I added this page into the navigation menu, I found it lost a CSS class “current-menu-item” which is helpful for me to highlight the current item in the menu.

Maybe I did something wrong or I should add/modify something somewhere. I’m totally clueless.I’m not very familiar with PHP5 and I’m new to WordPress.

Any help/suggestion is welcome! Thanks in advance.