This happened a while ago, but it's only just now making it to type.
When Elissa started having her bath with the boys, Ben asked why she had two bottoms. Since no obvious "willy" equivalent sprang to mind, I told him that she didn't have two bottoms, that she had a bottom and a vulva. This was digested without comment.
Many months later, at swimming while I was on my way back from Hong Kong, to be precise, Crash was changing Elissa's dirty nappy in the men's change room when Ben helpfully reminded him to "remember to clean her vulva" in his normal fog horn voice. I gather the remark attracted a wee bit of attention from all the men in the room. Crash may forgive me one day.