From England I would say your cheapest and best late December destination with warm weather and reasonable prices is going to be Phuket. There are some other Thai islands that might be a bit cheaper, but they would be more complicated to reach, and they’d have fewer activities for the boys. Here are some Phuket resorts good for families. You can see that the price range is wide from cheapest to most expensive. In late December it’s peak season so you’ll pay at or near the top of those ranges.
Located off the coast of East Africa, the Seychelles—an archipelago consisting of 115 islands—is about as close to paradise as you can get. Crystal clear water, lush jungles, and powder-fine sand make it the ultimate early winter escape, particularly for romantic getaways. Book one of the 30 breathtaking pool villas at Six Senses Zil Pasyon, which is among the region's top luxury resorts. The hotel is situated on a private island that offers prime territory for snorkeling, sailing, kayaking, and hiking—plus, the spa has scenic pavilion-style treatment rooms that are worth the trip alone. Health conscious guests will be particularly impressed by the property's new "Eat with Six Senses" program, which is intended to make travelers leave vacation without the guilt that comes with overindulging. There are four tailor-made treatment plans that focus on sleep, detoxing, fitness, and more, depending on the issues you prefer to address.