Solution:
The function name is different than what you have defined in add_action. Use the same name in the add_action and the function name:
add_action( 'wp_enqueue_scripts', 'hindrise_script');
function hindrise_script() {
wp_register_script('plugin-ajaxJs', CHILD_URL . '/js/demo.js', __FILE__);
wp_enqueue_script('plugin-ajaxJs');
wp_localize_script('plugin-ajaxJs', 'my_ajax', array('ajax_url' => admin_url('admin-ajax.php')));
}
Also in the JS file remove the quotes for the passing variable:
jQuery.ajax({
type: "post",
url: my_ajax.ajax_url,
data: { action: "data_fetch", keyword: input }
});