Flights book up quickly and there are just not the bargains that are associated with other parts of the globe. I like to plan ahead - we always book a flight as soon as we can. It takes about 9 hours to fly nonstop to Florida. Children become adults at 12 as far as most airlines are concerned. You can usually only book a flight up to 11 months in advance.
Most airlines now offer some kind of seat pre booking.
Take note however that these pre booked seats don't hold water if the flight is delayed, canceled, or changed at the 11th hour. This is the easiest option but not always the cheapest. There are a number of options that you might come across that land elsewhere in America. You then transfer to an internal flight to Orlando. There are pros and cons to this system.