Query children and parent title

i’m query child pages of a page ID. How can i echo the parent page where it says “PARENT TITLE ? Using <?php echo get_the_title( $ID ); ?> is not an option.

<?php

$args = array(
    'post_type'      => 'page', //write slug of post type
    'posts_per_page' => -1,
    'post_parent'    => '2', //place here id of your parent page
    'order'          => 'ASC',
    'orderby'        => 'menu_order'
 );


$childrens = new WP_Query( $args );

if ( $childrens->have_posts() ) : ?>


    <div class="content">
        <h2 class="big grid_3">PARENT TITLE</h2>
        <div class="grid_6 right">
        <?php while ( $childrens->have_posts() ) : $childrens->the_post(); ?>

        <div class="grid_2 left" id="teaser" >
            <p class="subtitle"></p>
            <h3><?php the_title(); ?></h3>
            <?php the_excerpt(); ?>
            <a class="cta_light" href="https://wordpress.stackexchange.com/questions/327307/<?php the_permalink(); ?>">Mehr erfahren</a>

        </div>

    </div>
    </div>
<?php
        endwhile; 
endif; 
wp_reset_query(); 
?>

Thanks!

I’m not entirely sure if I get your problem correctly, because of the “it’s not an option part”, but…

You get children pages of page with ID = 2, so it looks like you want to display the title of that page. If so, then this code will solve your problem:

<?php
    $args = array(
        'post_type'      => 'page', //write slug of post type
        'posts_per_page' => -1,
        'post_parent'    => '2', //place here id of your parent page
        'order'          => 'ASC',
        'orderby'        => 'menu_order'
    );

    $childrens = new WP_Query( $args );

    if ( $childrens->have_posts() ) :
?>
    <div class="content">
        <h2 class="big grid_3"><?php echo get_the_title(2); ?></h2>
        <div class="grid_6 right">
            <?php while ( $childrens->have_posts() ) : $childrens->the_post(); ?>
            <div class="grid_2 left" id="teaser" >
                <p class="subtitle"></p>
                <h3><?php the_title(); ?></h3>
                <?php the_excerpt(); ?>
                <a class="cta_light" href="https://wordpress.stackexchange.com/questions/327307/<?php the_permalink(); ?>">Mehr erfahren</a>
            </div>
            <?php endwhile; // <- I had to move that, it was in wrong place so you were closing more divs than you were opening in that loop ?>
        </div>
    </div><!-- .content -->
<?php
    endif; 
    // wp_reset_query();  <- there is no point in resetting query - you haven't changed it
    wp_reset_postdata(); // on the other hand you should reset postdata
?>

PS. Also take a look at my comments – I had to make some changes in your code, because it was generating incorrect HTML output.