Hi Lucy
You have probably worked this out already, but it's always nice to get a reply.
I have been in a similar situation very recently and looking at what years my children would go into in Jan 2016.
My 9 year old, in UK year 5 now and who will not be 10 until after April 2016, would start the new school year 2016/17 in SA in year 4
My 8 year old, in UK year 3 now and who will not be 9 until after April 2016, would start the new school year 2016/17 in year 3
This just reflects the later age at reception intake and the different school year start.
My 4 year old, currently in reception in the UK and in her second year at her school having been to nursery there, would only be going into Kindergarten in SA in January. It seems a big jump in years from the UK but actually, if we lived in Scotland rather than England the reception intake age is along similar lines in that children should be min 4yrs 6 months. In SA from what i have understood it is 4 yrs 9 months. In England it is 4yrs 1 day.
From what i can gather this means that they move onto secondary education a couple of years later and for a shorter time, but that sounds like a positive to me...rather than a little one off to high school the day after their 11th birthday
Hope that if i am mistaken, that someone with more knowledge jumps in.
Loads of luck with your move