Like I said, in actuality, this would likely not happen at all. Flight attendants are there to serve a safety function and a customer service function. If they see a mom with 2 kids sitting 2 rows behind exit row they are simply not going to pull the dad out of the exit row to go sit with his family...
In actuality, it happens quite often on SWA. As I recall, you have said that you have flown SWA once, correct? I fly SWA twice a month on average, and have done so for the past 12 years, and I know that they prefer to enforce this policy when the relationship is obvious. You're right that they won't pull the Dad out of the exit row, because he more than likely would not have gotten into it in the first place. SWA FA's stand at the exit row during boarding, and if they can see that an adult is supervising children as they board, they will tell that adult that he/she cannot sit in the exit row. I have heard it happen too many times to count. (In regard to the part about sending Dad to go sit with his family, you are also correct that they won't do that: they don't care where he sits as long as he isn't in an exit row seat. This isn't about keeping a family together; it is about the safety issue of keeping unsuitable passengers out of the exit row.)
The usual scenario where this happens goes like this: Dad and 1 school age kid board. Dad tells kid to sit in the aisle seat of the row in front of the exit row. Dad: "Got everything? Good. I'll be sitting right behind you if you need anything." FA: "Sir, I'm sorry, but since you're travelling with a child I can't let you sit in the exit row." Dad: "You're kidding, right?" FA: "No, sir, I'm sorry." Dad: "Come on Kyle, looks like we need to sit further back." They go to another row where Dad takes an aisle and puts Kyle in the center seat.
Now, will an adult get away with sitting in the exit row if he tells his kids that "once we get on the plane, pretend you don't know me"? Yeah, probably; but if you are talking about a family boarding with a carseat, the relationship is usually not so easy to hide, because they tend to make the relationship obvious in that situation, especially on SWA, where they will have preboarded to install the seat. (Which, of course, makes it a moot point; as they don't let preboarders sit in exit rows.)