I moved to NZ after doing half a term of Year 7 in the UK (moved to NZ in November). Could have done the last few weeks of primary school in NZ but got advised it would be like doing a GIANT step back. Year 7 in the UK = big school, lots of teachers, lots of new students to mingle with. So going back to primary school would have been a shock.
I ended up having that shock going into Year 7 at Intermediate school. One classroom teacher for the core subjects, reading books under the tree, teacher not really knowing how to do maths..... Because I'm a July baby, I shouldn't have really had the option to skip Year 7 and go into Year 8. I had the option because I was ahead of the other students in my class. But because by the time this all came into discussion, it was too late to skip (would have missed a term of Year 8!), I stayed in Year 7.
So it depends on when your children's birthdays are. If they are born February-June then maybe they can skip a year if everything is easy and they are mature enough to handle it.
But to be honest, your older child will probably end up finishing Year 8 in the UK in September and then doing a term or two of Year 8 in NZ at Intermediate school. They will then go on with their new friends to college/high school in January/February 2014. The other option would be only doing 2 terms of Year 9 in NZ (2013) and then going into Year 10 in 2014.....
Can't really give you much advice regarding your younger child.