Copyright info change in Theme Child PHP

You need to modify 2 functions not only 1 in your functions.php:

/**
 * Function to display footer content.
 *
 * @since 1.1.24
 * @access public
 */
//ADDED CHILD TO THE END OF THE NAME
function hestia_the_footer_content_child() {
    /**
     * Array holding all registered footer widgets areas
     */
    $hestia_footer_widgets_ids = array( 'footer-one-widgets', 'footer-two-widgets', 'footer-three-widgets' );
    $hestia_footer_class="col-md-4";
    $footer_has_widgets = false;
    $hestia_nr_footer_widgets = get_theme_mod( 'hestia_nr_footer_widgets','3' );

    /**
     *  Enabling alternative footer style
     */
    $footer_style="";
    $footer_style = get_theme_mod( 'hestia_alternative_footer_style', 0 );
    if ( isset( $footer_style ) && $footer_style == 0 ) {
        $footer_style="footer-black";
    } else {
        $footer_style="";
    }

    /**
     *  Get the widgets areas ids and class corresponding to the number selected by the user
     */
    if ( ! empty( $hestia_nr_footer_widgets ) ) {
        $hestia_footer_widgets_ids = array_slice( $hestia_footer_widgets_ids, 0, $hestia_nr_footer_widgets );
        switch ( $hestia_nr_footer_widgets ) {
            case 1:
                $hestia_footer_class="col-md-12";
                break;
            case 2:
                $hestia_footer_class="col-md-6";
                break;
            case 3:
                $hestia_footer_class="col-md-4";
                break;
            case 4:
                $hestia_footer_class="col-md-3";
                break;
        }
    }
    /**
     * Check if the selected footer widgets areas are not empty
     */
    if ( ! empty( $hestia_footer_widgets_ids ) ) {
        foreach ( $hestia_footer_widgets_ids as $hestia_footer_widget_item ) {
            $footer_has_widgets = is_active_sidebar( $hestia_footer_widget_item );
            if ( $footer_has_widgets ) {
                break;
            }
        }
    }

    hestia_before_footer_trigger();
    ?>
    <footer class="footer <?php echo esc_attr( $footer_style ); ?> footer-big">
        <?php hestia_before_footer_content_trigger(); ?>
        <div class="container">
            <?php
            if ( $footer_has_widgets ) {
            ?>
                <div class="content">
                    <div class="row">
                        <?php
                        if ( ! empty( $hestia_footer_widgets_ids ) ) {
                            foreach ( $hestia_footer_widgets_ids as $hestia_footer_widget_item ) {
                                if ( is_active_sidebar( $hestia_footer_widget_item ) ) {
                                    echo '<div class="' . $hestia_footer_class . '">';
                                        dynamic_sidebar( $hestia_footer_widget_item );
                                    echo '</div>';
                                }
                            }
                        }
                        ?>
                    </div>
                </div>
                <hr/>
                <?php
            }
            ?>
            <?php hestia_before_footer_widgets_trigger(); ?>
            <div class="hestia-bottom-footer-content">
                <?php
//ADDED CHILD THIS IS THE CALL TO THE COPYRIGHT
                hesta_bottom_footer_content_child();
                ?>
            </div>
            <?php hestia_after_footer_widgets_trigger(); ?>
        </div>
        <?php hestia_after_footer_content_trigger(); ?>
    </footer>
    <?php
    hestia_after_footer_trigger();
}
//WE CHANGE THE ACTION NAME AND THE CALL IS TO OUR FUNCTION NOW
add_action( 'hestia_do_footer_child', 'hestia_the_footer_content_child' );

    //THIS IS YOUR MODIFIED FUNCTION
    function hesta_bottom_footer_content_child( $is_callback = false ) {
    if ( ! $is_callback ) {
    ?>
        <div class="hestia-bottom-footer-content">
        <?php
    }
    $hestia_general_credits     = get_theme_mod(
        'hestia_general_credits',
        sprintf(
            /* translators: %1$s is Theme Name, %2$s is WordPress */
            esc_html__( 'Designed by HCL Design' ),
            sprintf(
                /* translators: %s is Theme name */
                '<a href="https://themeisle.com/themes/hestia/" target="_blank" rel="nofollow">%s</a>',
                esc_html__( 'Hestia', 'hestia' )
            ),
            /* translators: %s is WordPress */
            sprintf(
                '<a href="%1$s" rel="nofollow">%2$s</a>',
                esc_url( __( 'http://wordpress.org', 'hestia' ) ),
                esc_html__( 'WordPress', 'hestia' )
            )
        )
    );
    $hestia_copyright_alignment = get_theme_mod( 'hestia_copyright_alignment', 'right' );
    $menu_class="pull-left";
    $copyright_class="pull-right";
    switch ( $hestia_copyright_alignment ) {
        case 'left':
            $menu_class="pull-right";
            $copyright_class="pull-left";
            break;
        case 'center':
            $menu_class="hestia-center";
            $copyright_class="hestia-center";
    }
    wp_nav_menu(
        array(
            'theme_location' => 'footer',
            'depth'          => 1,
            'container'      => 'ul',
            'menu_class'     => 'footer-menu ' . esc_attr( $menu_class ),
        )
    );
        ?>
    <?php if ( ! empty( $hestia_general_credits ) || is_customize_preview() ) : ?>
        <div class="copyright <?php echo esc_attr( $copyright_class ); ?>">
            <?php echo wp_kses_post( $hestia_general_credits ); ?>
        </div>
    <?php endif; ?>
    <?php
    if ( ! $is_callback ) {
    ?>
        </div>
        <?php
    }
}

Now in footer.php we call our action:

<?php do_action( 'hestia_do_footer_child' ); ?>

Even if you modified the function, the actual function of the copyright was being called by another function, so you need to make your own version of that one too so it calls your modified function.