The first thing I would say is that Thailand is vastly inflated over Christmas & New Year. Over typical school holidays I imagine you would now be paying circa £800-£1000 for flights alone.

It would be a struggle to keep 10-14 days over this period below £2k per person.

Hi.

Thailand is very nice that time of the year, however the east-coast like Koh Samui which almost is good all year round can still have a touch of the monsoon in December still it use to be very good in the end of the month.

Phuket, and the complete west-coast is very good this time where the longer monsoon usually ends in November.

Where you should go is hard to say. For instance Phuket have many places, from extremely calm (even if many people doesn't know that) tio very crowded than also anything in between. Places like Kata tend to be very popular for families that wants a little bit of everything.

For calmer but yet not dead places Khao Lak might be a great option.

Wouldn't recommend going over Christmas to be honest - too expensive and too hot

Come to Koh Chang. Easy to get to by bus from Bangkok. Enough to keep kids busy. Not over developed like Phuket or Samui. And good weather too. ( But will be very busy at New Year as lots of Bangkokians head here for their NY break. )