I have flown to Florida on AirTran, SW, JB and Delta. Until Airtran and SW get their stuff together, I won't fly Airtran again. SW is fine..you just have to understand how it works. You are never 100% sure that you will all be seated together!! Airlines change equipment on occasion and the seating changes. Or they can change flights, combining two less full flights. Seats can, and do, change. SW seldom changes it's flights once they are announced. I always buy the EBCI but that's more for convenience...I hate having to worry about being near a computer, especially for my return flights. That way, I get into either the A boarding group or the low B group. If I'm traveling with just carryon bags, that's important. And you get 2 free checked bags. When traveling with kids, that hugely important!!
Love Jetblue. It's my airline of choice now. I love the individual tv's in the seat backs. Be sure to either bring your own ear buds or you can buy them for about $2 a pair. There are a lot of channels...especially for the kids!! They don't often change their flights...it does happen but not as often as with other airlines. And you get that first checked bag free.
I have flown Delta. But I finally got tired of paying for every checked bag and having to constantly monitor my flights. Delta makes a lot of changes to their flights if you have booked very early!! So, you may very well find yourself on a flight that just doesn't work out time-wise for you. And you could find yourselves split up vs in the same row.
In all reality? Choose the airline that works best for you. No airline is going to give you 100% assurance that your seats will not change. You just have to do due diligence in finding what works for your family as far as times, nearby airports and the airlines that fly out of that airport. Many people go by price alone...which isn't a bad idea. But price alone is hard if you need a specific airport and a specific time to leave.
Those that find themselves split up from their families usually booked very late, or didn't choose to pay the addtl fee to choose seats on those airlines that charge for that. I know that when I have flown Airtran, people that didn't pay to choose their seats, prior to arrival at the airport, found that their family had to split up..and that really upset them. It would seem that they didn't do their homework. Or, they decided to rely on the charity of strangers to give up their seats so as to enable the non-paying family to be seated together. That is never a good idea.!!!