Child page excerpt

get_the_excerpt() won’t work in your code if you add content to the main WYSIWYG editor instead of the excerpt field.

Check how get_the_excerpt() work and try to find out filter hook with get_the_excerpt hook name.

Please try the following code, hope you’ll get the expected result.

<div class="parent page">
    <?php
    global $post;
    $args = array(
        'parent'      => $post->ID,
        'post_type'   => 'page',
        'post_status' => 'publish'
    ); 
    $children = get_pages( $args );

    if ( ! empty( $children ) ) :
        ?>
        <div class="childcells"> 
            <?php
            foreach ( $children as $post ) : setup_postdata( $post );
                ?>
                <div class="childcell">
                    <?php if ( has_post_thumbnail() ) : ?>
                        <div class="thumbnail"><?php the_post_thumbnail( 'small-thumb' ); ?></div>
                    <?php endif; ?>
                    <div class="myclasstitle"><?php the_title(); ?></div>
                    <span class="desc"><?php echo get_post_meta( get_the_ID(), 'desc', true ); ?></span>
                    <div class="excerpt"><?php the_excerpt(); ?></div>
                    <a href="https://wordpress.stackexchange.com/questions/301420/<?php the_permalink(); ?>" rel="bookmark" title="<?php the_title_attribute(); ?>">Read more</a>
                </div>
                <?php
            endforeach;
            wp_reset_postdata();
            ?>
        </div>
    <?php endif; ?>
</div>