Unless your child has an exam in January or February then give them a break, let them enjoy Christmas without the weight of testing, homework and all the rest of the stuff its tempting to do whilst they are off from school.
.If you want them to do some work then make it fun; find some videos on You Tube which will teach them something, as with the Chuckra verbal reasoning videos, designed for the 11 plus and still useful for those going for entrance exams in year 7.
How about the holiday games on coolmaths4kids which will stretch their minds, get them working logically and sequentially as well as having great fun.
Then there are the puzzle books with spot the difference, word searches, crosswords, maths puzzles; all will stimulate their brains but in a way which doesn't feel like work.
There will be work to do no doubt, but let them relax and remember, they don't go back until Monday 6th January, so use the time after Christmas and then a couple of days in January to get them back into the swing of it, they will go back to school relaxed and refreshed.
One major reason children have regular holiday time away from the pressures of school is to forget the stuff they didn't quite get and absorb what they did. Let's give them that period because if they are sitting 11+ in September there wont be that many breaks after Easter!!
To all my students, past and present, have a great Christmas and fabulous New Year celebrations.