Reloading page with a query string upon login for admins

Try using the filter hook:

function user1462_login_redirect( $redirect_to, $request, $user ) {
    global $user;

    if ( isset( $user->roles ) && is_array( $user->roles ) ) {

        if ( in_array( 'administrator', $user->roles ) ) {
            return $redirect_to . '?message=hiadmin';
        } else {
            return home_url();
        }
    } else {
        return $redirect_to;
    }
}
add_filter( 'login_redirect', 'user1462_login_redirect', 10, 3 );