My thought isn't that the creation of child would break the soul/create a Horcrux, but rather that it would counter, at least in part, the evil of the Horcrux already inside Harry. Harry and Voldemort's souls are very intertwined while the Horcrux is still in him. So the superior act of love of making a child would affect Harry's soul for the better - and the evil, damaged part of Voldemort's soul for the worse. Think of Harry's soul as a positive number - the addition of another positive number would make it greater. Voldemort's (Horcrux-ed soul, dwelling within Harry) would be a negative number, so the addition of a positive number would actually dwindle it(s negativity.) i.e. -10 + 4 = -6
Here's my first thought - the natural state of one's soul is to be whole. A broken soul, a soul split into pieces, is no longer natural, no longer whole. This strikes me as a ... bad thing, and hence why doing something bad, something evil, does such a thing to a person's soul. Killing in unnatural, and does unnatural things to the killer - rips their soul in two. This is not desirable at all.
So I think that an act of love, of creating a child - something so natural - shouldn't be something that splits the soul and creates something unnatural. It just doesn't seem right, it seems the opposite of what nature intended. Does that make sense? A Horcrux is Dark magic - birth doesn't seem the way to bring about something so terrible.
Perhaps I wasn't clear. Imagine an AU story where Harry has children before the Final Battle with Voldemort. I'm talking about love splitting the evil soul dwelling in Harry - and thereby accidentally passing down the Horcrux to the child. Not genetically, but by the magic that would occur when the act of love split the soul - just as when Harry was accidentally implanted with (for lack of a better term) the Horcrux in the first place.
And when it comes to the original post, and the question of Harry passing on Voldemort's Horcrux to his children via his own soul: I thought that Horcrux was destroyed, gone from him forever. Now, I could see a good fic bringing it back, but I can't see it being passed down genetically, so to speak. Unless, of course, there was some other magic invovled. Like, a curse or a potion that was used to enable it to happen. Then I could see something like that working.