Add custom options to an ACF field.

wp_acf_load_field_choices.php

<?php
/**
 *  ACF Load Field Choices.
 *
 *  @param   array  $field  The field that we are going to add options to
 *  @return  array  $field  The field we modified
 */
function acf_load_field_choices( $field ) {
    // Reset choices
    $field['choices'] = array();

    // Get all the child pages
    $child_pages = get_children( array( 'post_parent' => 1, 'post_type' => 'page' ) );

    // Add our child pages as options
    foreach( $child_pages as $child ) {
        $field['choices'][$child->post_name] = $child->post_title;
    }

    return $field;
}
add_filter( 'acf/load_field/name=field_name', 'acf_load_field_choices' );