As far as I've noticed from the programming, they do have different personalities like a random generation or something that depends on a bunch of factors. Even the clothing could have something to do with it as opposed to the heads, the only way we'd know is if a staff member told us the facts

I have had some rather persistently argumentative couples but they always give in in the end and have some lovely children. Unfortunately, these couples make it a challenge to reach more than 3 or 4 children but I'm glad to get what I can from them.

Best of luck with your families!