As I climb on my soapbox again, I feel, all EBCI should be able to board before family boarding. If you happen to get A59, your $25 got you a lot more than someone who gets B 1. I've been on Disney SW flights where it looked like 1/3 of the plane was families with young kids.
With open seating that is not possible due to FAA regulations, because the seating of young children is restricted to certain positions and proximity, so the airline has to make sure that they have "legal" seating positions available when they board. That might not happen if all of the EBCI holders were allowed on first.
US law guarantees the right to use an aircraft-approved child safety seat if the "berth" (the seat) is paid for and the child weighs less than 48 lbs. There are also FAA regulations prohibiting the placement of child safety seats in certain seating positions, most notably both the exit rows and the rows immediately in front of and behind the exit rows, and it is also Federal law that a carseat may not be placed where it might "impede passenger egress in an emergency" -- which most airlines interpret as requiring placement in window positions only (or window and middle if you have 2 of them with only one adult.) The adult responsible for getting the child out of the seat in the event of an emergency evac must be seated next to it.
It's also Federal law that children under the age of 6 must be seated contiguously to a member of their party who is at least 14 years old, because children that age typically cannot read and will not know what to do in an emergency unless someone older assists them.
All that said, SWA has recently really dropped the ball on most routes on policing preboarding and family boarding. They have been allowing much more leeway than is required by FAA regulations, and it has been not only gumming up the works, but causing annoyance for other passengers.
I remember when SWA switched "family" boarding from preboarding to between A-B groups. For a good 6 months either side of that decision the gate agents really cracked down and refused to let more than two adults board with younger kids, and also refused to let teens join younger siblings for special boarding unless they also claimed disability. I'm guessing this resulted in ugly confrontations in some airports (at mine people quietly grumbled but complied), and the legal teams probably decided that the litigation risk for employee injury claims was too high, so it was stopped.
What I can tell you is that the advent of EBCI was the catalyst that caused conditions to detiorate. That system was always ripe for abuse, and I wish they had never started offering it if they were not prepared to police egregious seat-saving. SWA has placards that they give to COS who purchase a second seat, and my feeling has always been that if there isn't one of these in the seat, then it is free to be taken, but I try to be gracious about taking seats if people are gracious about asking to save them.
So, do I "save" seats when I travel with family and I'm quicker on the button than my DH is? (We do not purchase EBCI for anyone, and we often have separate reservations.) Yes, in that I'll go to the far back and sit in an aisle seat. If someone asks I'll politely tell them that DH & DD are boarding several slots behind me and that I'd appreciate it if they could let me hold the 2 seats for them, but I don't insist, and if they want them then I'm fine with that.
I don't blame people for getting angry at passive-aggressive seat-saving tactics like putting a 6 yo in a middle seat to save an entire row, or spreading belongings around. If your child is not prepared to sit between strangers, that's not only a crappy thing to do in general, but it's cruel to your own child. If you are going to attempt to save seats, limit yourself to no more than 2 seats for actual passengers (no phantoms to keep the middle seat open), go to the very last open row, *own* what you are doing, and graciously concede if challenged. No cursing, no huffing under your breath and rolling your eyes, no whining. And no purchasing EBCI for just one member of the party to gain an unfair advantage.