To be fair, the reason why later Star Trek ships abandoned the concept of families on board has been attributed to perhaps being exactly because the Battle of Wolf 359 acted as such an eye-opener. The idea of having families on board ("generation ships") was perhaps something Starfleet was experimenting with at the time of TNG, until incidents like the Borg incursion shown how foolhardy this could be.
Having said that, I agree that the armada that faced the Borg Cube knew they were going into battle, and the knowledge they had about the Borg meant they knew it might not be very pretty for them. So there really is no excuse for them not having off-loaded all of their non-essential personnel (like families) the moment that the mission briefing came through.
Same goes for the Enterprise, although as The Old Mixer says at least Enterprise has allegedly got a contingency plan (the saucer-sepper, not that Riker used it that way in the event!).
The best excuse I can offer is: the Titantic went out despite grave concerns being raised before she set off, about the massive gulf between the amount of passengers booked on her maiden voyage vs the paltry number of lifeboats. Sometimes it takes a big catastrophe in order to open the eyes of people to the most obvious of things. Sometimes idealism outweighs common sense, and it'd take families dying aboard ships like Saratoga to convince Starfleet maybe it isn't such a keen idea to put civilians on these things...
Yeah I mentioned this in a another thread, they even had a kid on an oberth class ship on one ep. (though to be fair, it was an S.S. ship not U.S.S. ship if memory serves, so it was prob. operated by a civilian group.) I just think Starfleet was way over optimistic at the time. They probably thought that peace with the klingons and the romulans while being dangerous were not threatening war, that eventually they would make peace with the romulans and major conflicts would be a thing of the past.