Load File (Function & Variable Value) from Child Theme

I would suggest to use the WordPress Customize API. There you can set as many fields as you want and all will be nicely shown in the theme’s Customize menu.

<?php
/* New Section in Customize */
add_action( 'customize_register','new_customize_option');
function new_customize_option( $wp_customize ) {

    /* --------------REGISTERING NEW SECTION --------------- */


    $wp_customize->add_section( 'rewp_logo' , array(
    'title'      => __( 'Copywrite fields', 'textdomain' ),
    'priority'   => 260,
    ) );

    /* -------------- S E T T I N G S --------------- */


    /* Setting: Set a text field   */
    $wp_customize->add_setting( 'rewp-text' , array(
    'default'     => 'Copywrite 2017',
    'capability'  => 'edit_theme_options',
    ) );

    /* -------------- C O N T R O L S --------------- */

    /* Control: Upload Background */
    /* Control: Set a link */
    $wp_customize->add_control( new WP_Customize_Control( $wp_customize, 'rewp-text', array(
    'label'     => __( 'Copywrite text', 'textdomain' ),
    'section'   => 'rewp_logo',
    'settings'  => 'rewp-text',
    'type'      => 'text',
    'priority'  => 1
    ) ) );

} ?>

Then you can use it like:

$mods = get_theme_mods();
echo $mods['rewp-text'];

The suggestion of @Rishabh will also work but mind that if this page gets deleted by accident then you will need to create a new one and edit the code again.

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