Check if the school is doing mock exams in your DC's subjects in December, or modules in January. The modules can be a large part of the final grade so you really don't want them to miss the teaching and practice/revision done in December. If there are no mocks and no modules you may be OK, but your DC will have to do some serious work to catch up on three weeks' missed work and you may end up kicking yourselves if they do badly in the summer, or just miss a grade by a few marks.
It's not an ideal time to be missing school. At most schools there will be the first set of AS modules in January, so he would miss preparation and probably some of the syllabus teaching for those with very little time to make up the lost work before the exams. Some schools sit all the AS modules in June, so exam preparation wouldn't be an issue, but December is about 3 weeks school in the most learning intensive term, so he would miss a lot of work there too. Given a lot of children do find the transition to A level quite tough, it might be an idea to leave him at home (staying with a friend/relative) for the first part of December and have him join you once the term is actually over.
Next year, we hope to visit my parents in NZ for the whole of December. By this time our eldest would of just started A Levels (in Sept) but we are concerned that taking December out would mean missing modules etc.
Is there anyone who has kids who started A Levels in September that can offer their opinion on whether December is a good or bad time to go away? I appreciate that all courses are different but was just after the general feeling.