Well huh. There were a couple of sites that attributed it to Martin Luther's "The Estate of Marriage," but I looked at it and coming from that would mean more than a bad translation -- it would be an ideologically driven paraphrase. I'm assuming it having appeared in an Armstrong book means that it was written closer to as quoted elsewhere, but you never know.
From The Estate.. :
(The following paragraph was all about the virtues of a father washing his child's diapers.)