The reason to board small children before the B BPs are that:
1) It sometimes takes us a long time to get a car seat belted into an airline seat, get the toddler in it, store our stuff, and get settled. Do you really want to sit and wait while you watch me go through the whole routine?
2) If a family boards late, FAs wind up having to re-seat alot of people to accomodate a child who needs a parent near. Hassle for the FA, hassle for the re-seated, hassle for the passengers who have to wait while all this goes on.
Should families pay the EBCI? Heck, yes! I will. Will all families? No, some because they don't want to, some because they can't afford to, some because they don't know about it.
BTW, you are not paying to get on before other people. You are paying so that you do not have to hover by your computer at the 24 hour mark. You are paying for an automatic BP assignment. You are paying for convenience, not a particular spot in line.
And this is exactly why I think SW should hold the back 1/4 of the plane for families. They can board first, and go back and take their time getting settled. No one would have to wait for them to get their seats buckled in, their massive amounts of gear stowed (yes, I've traveled with a youngster before), and get ready. They can board first, but not take up space while getting settled in the front or middle of the plane.
Sure seems like it would be a more equitable way of doing things...especially if people start paying extra for early bird checkin but end up with B bp numbers. That really doesn't seem fair to most of us. We do understand that families need more time to get situated, and most of us have no issue at all with families boarding inbetween the As and the Bs. But once someone has to pay for something and finds someone else doesn't have to pay for the same thing, it's going to get murky...and not so pleasant.
Here's a scenario.....
My family of 5 (not my case, just a scenario) is travling to MCO during a busy vacation time. We have no children 4 and younger with us, so don't qualify for inbetween A and B boarding. But, I realize it's going to be a busy time to travel so I decide to pay the extra $10 so I can get a bit of a jump on the seating. I have now paid an addtl $50..yes, for early checkin, not early boarding. But, because there are so many others traveling, I end up with B 1,2, 3, 4 and 5 spots. No problem...we're good to go. But, when we get to the airport and find that there are about 25 people who qualify for boarding inbetween A and B groups, well, that's going to annoy me. I paid that $50...they didn't. Now, they get to all board, with their families (and they do tend to board in groups that really aren't immediate family!) and by the time they are onboard, there is no way my family can now sit anywhere close to each other...maybe 2 in row 12, another 2 in row 20, and who knows where Dad is going!!
That would be my issue. And that's why I think that they need to have those who really need to preboard, go ahead and really preboard, but head to the rear of the plane so that others can more quickly board, without having to work around the familes.
Not to mention....there might be less complaining about what others' children are doing...you pretty much know where the youngsters are going to be on the plane.
, i'll restate that it's highly likely that many passengers will request the early board. While is probably more geared to DISability board as to who is eligible, i do hope that SWs staff is equipped with guidelines as to determining eligibility. Many passenger's disabiities r hidden (low vision, balance issues, neuropathy, especially in the more senior among us). Im thinking (in lieu of dr's note which i think violates privacy issues anyway) bringing one's HC parking placard would be a good idea. 
