it is permitted to order the child to take off his cloths to give him a spanking. it is not a problem of "tznius"?

Physical striking of one’s child is permissible by Halacha however a parent should be genuinely considering this method of punishing or educating, as I believe, for most cases nowadays it might have the opposite effect. In any event, parents must not embarrass their children; spanking a naked kid is humiliating and as such - prohibited.