I was under the impression that most airlines will strive to get families together anyway, hence why you often get seat changes at the gate or couples or groups of adults who have paid for seats online in advance are suddenly told at the airport that they are no longer in the seats the parted cash for. I know that any airlines I've worked with have usually pre-reserved seats for families before check in opens, provided they have got the correct information from tour operators/travel agents etc in advance.
I am not sure I totally agree that families should sit together no matter what. I mean children should never be parted from an adult, even if it means moving others, but if you have 2 sets of 2 seats left and a family of 4 turn up at -40 then they should accept they will most likely not be sitting together. That seems perfectly fair to me as if you start moving people to accommodate late running families then you are just going to upset other people, who may be regular, loyal customers, which I think is unfair if a perfectly reasonable assignment can be given out of the remaining seats.
I remember being stuck behind a rather irate woman at check in once who was kicking off because the only seats available for her and her daughter (10 years old) were 2 aisle seats over from one another (10C and D if I remember correctly). She claimed her daughter would be "distraught" at being "so far away" from her mother (the girl, might I add looked as if she wanted to be sat as far away as possible from her mother). The check in agent was talking to a brick wall explaining that these were the closest seats left together and that they could have check in online and got seats at least 48 hours in advance, as was stated on their email (which the lady had printed off). She was also told she could ask to swap on board but the irate woman just would not give in until a manager was called over!