dynamic add_action according to child pages (for homepage control)

The use of add_action() seems backwards to me. A better approach would be to hook homepage_content() into the homepage hook and then just output each section within the one hook:

function homepage_content() {
    $the_id = 5;
    $item = get_post( $the_id );

    $page_tree_array = get_pages( array(
        'child_of' => $the_id
    ) );

    foreach( $page_tree_array as $item ) {
        $id = $item->ID;
        $slug = $item->post_name;
        $title = apply_filters( 'the_title', $item->post_title );
        $content = apply_filters( 'the_content', $item->post_content );
        ?>

        <article id="post-<?php echo $id; ?>" <?php post_class( 'homepage-content' ); ?>>
            <h1><?php echo $title; ?></h1>
            <?php echo $content; ?>
        </article>

        <?php
    }
}
add_action( 'homepage', 'homepage_content' );

There’s some other changes you could make that would help too, but I left out to focus on the use of add_action():

• Get the homepage ID dynamically with get_option( 'page_on_front' ).
• Setting $item to the get_post() value for the ID seems unnecessary, since you never use it.
• Use setup_postdata() for each section so that you can use template tags like the_title() and the_content().
• Using setup_postdata() also means you could put the homepage section template into a separate file and call it with get_template_part().

Taking all that into account and your homepage_content() function would look like this:

function homepage_content() {
    global $post;

    $child_pages = get_pages( array(
        'child_of' => get_option( 'page_on_front' ),
    ) );

    foreach ( $child_pages as $post ) {
        setup_postdata( $post );
        get_template_part( 'partials/homepage-content' );
    }

    wp_reset_postdata();
}
add_action( 'homepage', 'homepage_content' );

Then create partials/homepage-content.php as the template for each section. Since setup_postdata() was used you can use the proper template tags:

<article id="post-<?php the_ID(); ?>" <?php post_class( 'homepage-content' ); ?>>
    <h1><?php the_title(); ?></h1>
    <?php the_content(); ?>
</article>