Вещи, о которых мало кто имеет верное представление

In the original there was a humorous turn of phrase, which literally would be translated 'That is to say, I had a child somewhere, but my wife had no interest in it', the interest being like that in a commercial operation - as who should say, I had a paper mill somewhere, but she had no share in the profits. My difficulty was that if I said 'interest' in English, the reader might suppose that the wife knew about the child and did not care - but I think that he never told her at all (why would he inform his wife of his illegitimate children?).