You could join everyone else and come to South Florida. The weather is usually much nicer that time of year. The Keys also have nice weather, but I would get bored spending a whole week there, unless most of that time was spent in Key West. We're not really fans of the upper Keys, but it really depends on what you enjoy doing.
How was it? I am really having trouble making a decision on this for some reason.
You've received a lot of good suggestions. I can see why it would be difficult to choose one. It really depends on how active you want to be & what type of entertainment you're looking for. Are you looking for a laidback, beach vacation or do you want to spend a few hours at the beach, then have a lot of other things to do?
) Ft. Lauderdale to Miami & the beach towns in between are basically one long beach with unlimited things to do. You can probably catch a pro sports game of some type, if your family is interested. The weather is very nice in December. Be warned that the traffic can be horrendous. If having a lot to do that doesn't require being in or on the water most of the time, interests you, check out Margaritaville Hotel on Hollywood Beach. https://www.margaritavilleresorts.com/margaritaville-beach-resort-hollywood-beach It's between the two cities & the area is usually more laidback than Miami beaches. 
Alaska flies direct from Seattle to Orlando. That would be the easiest option if you go to Disney first and road trip to a beach afterward.
I agree that flying into Orlando would be the best option, but I would recommend flying out of a city closer to their beach destination. To me, any of those options would be better than driving back to Orlando & flying out of MCO. IMO, MCO is the worst airport in Florida.
Curious, what area of NSB is your family condo? We are leaving Saturday for NSB. We are staying further down, Smyrna Beach Club. How bad is the sand down that way?
We were right at 44 and 3rd Ave. Sand was pretty hard packed, but easily accessible during low tide (9/10am - 5pm). Plenty of space to bring a chair and lounge, not great for sand castles. We missed our evening beach walks and hunts for crabs, btu still had a great time. I hope the rip currents are better for you than they were for us! Yellow/Red flags the entire weekend.
I have to ditto Marco, SW FL. I am from Lee County. In the winter, we are definitely warmer down here too.
Alaska Air also flies nonstop to Tampa.
TPA is a great airport. If I was going to rent a car anyway, I'd definitely fly in & out of there to avoid MCO.
Have not been in MANY years. My Mom had "timeshare" type condo there on intercoastal side. It was a few minute walk to beach across street. We had so much trouble with beach front properties harassing us for being on the beach. I know that this is becoming a huge problem in other areas now. Is that still the case, if so one should stay ON the beach.
