Like AmbitiousAmoeba mentioned, you’ll need to perform a few get_pages calls to lookup the pages for each child, merge the results together, and iterate over those results.

I worked up an example for you, because it’s fiddly trying to merge arrays when they have matching indexes, the only thing you should need modify are the child_of parameters, and of course add back in your surrounding HTML..

Updated example

Uses array_merge instead, i mistaken thought it wouldn’t work for with numeric keys(it does though, as Rarst kindly pointed out in the comments)..

$args = array( 
     'child_of'    => 174,
     'sort_column' => 'post_date',
     'sort_order'  => 'desc'
);
$child_pages = get_pages( $args );

$args['child_of'] = 143;
$second_page_group = get_pages( $args );
$child_pages = array_merge( $child_pages, $second_page_group );

$args['child_of'] = 146;
$third_page_group = get_pages( $args );
$child_pages = array_merge( $child_pages, $third_page_group ); 

unset( $second_page_group, $third_page_group, $args );

if( !empty( $child_pages ) )
    foreach( $child_pages as $_my_page ) {
        $title   = apply_filters( 'the_title', $_my_page->post_title );
        //$content = apply_filters( 'the_content', $_my_page->post_content );
        //$content = str_replace( ']]>', ']]>', $content );
        $link    = apply_filters( 'the_permalink', get_permalink( $_my_page->ID ) );
        ?>
        <div class="thumb-container">
            <div class="thumb"><a href="https://wordpress.stackexchange.com/questions/5580/<?php echo $link ?>"> <?php //echo get_image( "thumbnail", 1, 1, 1, $_my_page->ID );?></a></div>
            <p class="smalltitle"><?php echo $title ?></p>
        </div>
        <?php
    }

Old example

Might aswell leave the old example in place

// Setup base args for each get_pages call
$args = array( 
    'child_of'    => 174,
    'sort_column' => 'post_date',
    'sort_order'  => 'desc'
);
// Perform the first query to get pages
$child_pages = get_pages( $args );
// Count the results
$c = count( $child_pages );
// Update the child_of arg to another ID
$args['child_of'] = 143;
// Perform the second call to get children pages
$second_page_group = get_pages( $args );

// If there were pages in the first group
if( $c )
    // Add them into the first array returned
    foreach( $second_page_group as $k => $p ) {
        $child_pages[$c] = $p;
        $c++;
    }
else
    // Else fill the array with the results of the second query
    $child_pages = $second_page_group;
// Cleanup
unset($second_page_group);
// Count the results again
$c = count( $child_pages );
// Update the child_of arg to another ID before the final get_pages call
$args['child_of'] = 146;
// Make the third call
$third_page_group = get_pages( $args );
// If we have results, add these into that array
if( $c )
    foreach( $third_page_group as $k => $p ) {
        $child_pages[$c] = $p;
        $c++;
    }
else
    // Else fill the array with the results of the final query
    $child_pages = $third_page_group;
// Cleanup
unset($third_page_group);

if( !empty( $child_pages ) )
    foreach( $child_pages as $_my_page ) {
        $title   = apply_filters( 'the_title', $_my_page->post_title );
        //$content = apply_filters( 'the_content', $_my_page->post_content );
        //$content = str_replace( ']]>', ']]>', $content );
        $link    = apply_filters( 'the_permalink', get_permalink( $_my_page->ID ) );
        ?>
        <div class="thumb-container">
            <div class="thumb"><a href="https://wordpress.stackexchange.com/questions/5580/<?php echo $link ?>"> <?php echo get_image( "thumbnail", 1, 1, 1, $_my_page->ID );?></a></div>
            <p class="smalltitle"><?php echo $title ?></p>
        </div>
        <?php
    }

Hope that helps…