Wow! Is it really that big of a deal? How many people even take advantage of it on a flight where it's offered? As a male I can easily handle carrying an infant on board along with a carry-on and diaper bag. For many women (some men) traveling alone with a child or children can be tough. I think the pre board is not so much for putting the bag/bags in the overhead (which takes like 5 seconds) How about getting themselves and the child(ren) settled into the seat and buckled in? (Long after they are no longer in the aisle)

Now I totally agree that they should wait to get off the plane until last. This is where the traveler needs to be responsible and allow PLENTY of time between any connecting flights, minus any delays that were not any fault of their own. If time is not an issue, then the courtesy needs to be returned while they allow everyone else off first.

I've seen way more aisle traffic jams caused by some fool trying to make his way up to the front of the plane from the back for whatever reason while people are still boarding, than I have from a family that had pre boarded. Hopefully they used the "pre" to get into their seats and not to just lounge in the aisle.

As far as what I have personally observed:
I used to fly primarily NW and generally always heard them make the announcement for every flight. The first time I flew AA, I happened to have my kids with me, and noticed they did not make an announcement. Out of curiosity I asked the GA if they do pre boarding for small children, she politely said they did not.

I have no problem with airlines if they do not do it. I know I am going to get on the plane at some point, so I could care less. I also have no issue with it if they do allow it. To me it's just common courtesy. Now that courtesy can certainly be abused by an ignorant person, but that's a whole other convo.