In my first save, I married my spares out into premade households, and I think it may be possible that some of them had babies later without my prompting. Like, once or twice at most. I usually had them try for baby before the wedding party ended so most babies were because of that, but way back when the game was young I think there was a baby I couldn't explain.
The families you haven't touched at all will not change in any way, except to age if you have the game set to aging on for all sims.
I used to go into the Manage Households and just create children combining the genetics of sims who lived together to keep the townie/premade generations going if I liked the families enough.