We always use EBCI so we can sit together. It is worth it to us and I don't like to stress over the small things when we travel.
With EBCI vs. not, it will depend on several things. First, how busy is the flight. If it is a sold out flight then it is more important and sometimes fully booked flights do not have family boarding. However, your kids are older and FB is not an option. Second, at this point it depends on how many have already paid the EBCI. The A group and part of the B group may already have been paid for since it is so close to your flight. The boarding position depends on when you purchase, first come first served from my understanding. More and more people are paying for the early boarding, especially on family oriented flights.
If you want to be together then I say use the EBCI. If you are OK with splitting 2 here, 2 there and 2 somewhere else, then go with the 24 hour free check in. Just remember that on a full flight some could be filling in single seats. It's hard to give you an exact answer without knowing if the flight is full and how many EBCI people are already in line.
It doesn't look like the flight is full, they are still selling tickets for it. I did buy the EBCI and I am fine if we have to sit 3 and 3. Thanks