Yes the weather can be cold, but like the rest of Europe you will find it can be variable, nothing is certain on this subject.
Do you realise that everything will be closed on Christmas day and New Years Day too. Plus things start to close early on the day before these public holidays too. Some areas will also have also have public holidays during this time too add to the days of closure.
You may find a lot of restaurants closed on 25th Dec and 1st Jan so eating out may prove hard.
Now how do you want to travel? and what are your interests?

I've spent several Christmases in Spain over the years. The country does not shut down on Christmas Day: trains and buses run, and some bars and restaurants are open. Spaniards normally have a family meal at home on Christmas Eve, and very few restaurants are open then.
Madrid is generally cold in December, but southern Spain and the coasts can be pleasantly mild. The weather is not predictable so you should be prepared for almost anything.

I think you'd like Spain during the Christmas season, which doesn't end until 6 Jan. Madrid may be a little cold for you, being from Miami, but the city is alive and lit up during the season. Always a good excuse to buy a new coat. I’d do a few days in Madrid and then head to Barcelona, where the weather should be fairly mild. It's a Mediterranean climate. The last time we were there for New Years is was in the low 60’s during the day and comfortable at night.

I spent the month of January 2000 in Madrid, with trips to Sevilla and Granada, finishing with a couple of days in Barcelona.

Granted that I chose Spain because I was sick of 10 degree weather, but I found Madrid to be pretty comfortable. The Spaniards were all buttoned up, but I didn't even zip my coat. There was no snow, and there was plenty of sunshine.

Jeff thank you for confirming what I said about hard to find restaurants etc. Yes public transport maybe running but it is usually on a reduced service. Museums and public offices are closed so no visiting these sorts of places.

crowds will not be insane, weather will probably be good in the south, cold in Madrid; if you like cold even snow how about this for a rough itinerary:

Pre-Christmas and Christmas Day in Madrid: lively up to the 25th then on the day you can dine big style somewhere at lunchtime and just snack at your hotel in the evening when there may not be so many eating options. Everything back to normal on the 26th (not a holiday).

Depending on how much time you have: from there head south to Cordoba for a couple of nights, then Granada/Sierra Nevada for the alhambra/city experience then some serious snow. Even if you don't ski there are options like cross-countyr skiing (with classes), husky sled rides, nature walks, or just buy a couple of kid sledges and slide down a slope.
From there, Sevilla for New Year. You will find places to eat at lunchtime on NY Eve/NY Day and can just fillup in the tapas bars in the evenings. From there, back to Madrid (if you have enough time) for the Three Kings (the big spanish present-giving celebration and parades) on the 5th/6th.
Alternatively you could fly from Sevilla to Barcelona for a few days but I think Madrid is the place to be for Los Reyes (Three Kings).

forgot to add: Madrid is not bad for kids - big park, movie theatres with VO (original version) films etc; Barcelona has plenty to do, Sevilla is a living theme park and Cordoba is surprisingly good for kids - there's a great adventure/activity park, a zoo, some good trips close to town, lots to see walking around

Thanks everyone for your great advice, now we aren't sure if we want to tackle this trip in the winter or wait till next year. I will save all this info for then.

Just found this thread - I am also from Miami Beach and planning to take my kids to Spain for christmas break this year. Did you ever take the trip? Do you have any suggestions for a family of four, kids ages 10 & 14? I would appreciate any info you could share. Thanks!

We actually went this summer and it was fantastic!! I think the winter trip would also be great especially in the South and Barcelona where it wont be freezing!!! We loved every minute of Spain!!! Mt itinerary is on here somewhere, I never got around to the trip report, but Ill get to it one of these days. Click my name and youll find all my questions and answers I received.

We did ten days over New Years last year. It was cool and rainy much of the time. Fly into Madrid and home from Malaga with stops at Cordoba, Seville, and Granada. New Years was in Seville. And, yes, Seville did shut down for New Years eve and day. The is no gala New Years Eve celebration in Seville like you find in the US. With the exception of a couple restaurants in large hotels, everything was shut down and remained so on New Years day. We prepared by buying enough food for those two days.

The Madrid Royal Palace has a side/annex museum that is the armory. Lots of knightly garb there including pint size prince's armour. Kind of interesting for kids. New Years in the Plaza Major you eat white grapes as the bells toll..12 for good luck throughout the new year!! Madrid was not cold when we were there for Xmas/New Years but who knows..Layers are the garb of the day.