Maybe it was just me, but Nowi's childish ways seemed to be at least partially an act to get people to let down their guard. If you bother to S-rank her with anyone, she makes it clear that she's perfectly aware of what marriage is and what it means, whereas someone with a child's mind simply wouldn't get it.
Or maybe not. Maybe that's something the localization added so that you didn't have a bunch of grown men having children with someone who was, in every way except chronologically, a child herself (and all the squick that implies).
* Sirus shrugs
Her daughter didn't seem to be quite so childish, from what little I saw of her.