you won't find any place warm enough in December in Europe.. you can maybe think of Finland-lapland ( Santa Claus village, northern lights etc..) because anyways its going to be cold , go to a place which is meant to be visited in winter.. 12 Reasons to Visit Finnish Lapland in Winter | TravelGeekery
Gibraltar or Seville in Spain are warm enough to go in December (17-18 c temperature). Every where else in Europe will be cold. San Diego, CA will be about same temperature and there is tons of stuff to do with the kids. South America and Australia will have summer in December if you want real warm weather.
Australia and NZ is the best place for December vacation if you want the weather to be very warm. South America is also somewhat similar, but not sure about the vacation destinations there. You may search a bit. Valencia in Spain is a great spot, and the weather won't be too cold there during december. We had a great time a few years back during December, so recommending it
