I've lived in Florida for 30 years. Weather is very hit or miss this time of year. It was chilly and rainy last week, sunny and shorts weather over the weekend, back to heavy rain yesterday, and so on. I do not go in the ocean or even to the beach really between September-March. The only people you see in the ocean at this time are fellow tourists.
1. Cruise - I'm not a fan of cruises in general, but this may be a great choice if traveling with a toddler. You will get the warmer weather and some beach time in port, and you will have structured child care available so you can enjoy some time alone. Lots of activities for all of you. You may be able to get some good last minute deals too.
2. Siesta Key/Sarasota area - I grew up about 45 minutes north of here. Siesta Key has been ranked the #1 beach in America at times. Sugar soft sand and great Gulf Coast sunsets. December is not an optimal time to visit (cooler weather, snowbirds starting to come down), but if you just want a very laidback trip with pretty scenery, this could work. Find a place with a heated pool and hope it's not rainy.
3. Orlando-Miami-Keys roadtrip - no offense, but sounds like my idea of hell with a baby
You will spend most of your time in the car. Orlando and Miami are both notorious for horrible traffic. You will get the warmer weather in Miami but I think of Miami as more of an adult playground. Expect topless and thongs on the beach. The Keys are very pretty but keep in mind it's a chain of islands. There is very little "beach" to speak of in Key West, but you could go to Bahia Honda (?) state park. The Keys are also extremely expensive. I'm often tempted to go there, but when I price it out, it always ends up being cheaper to fly to somewhere else in the Caribbean.