Here are the Term times from the Department of Education Website
You'll be coming in half way through the school year as we did. (we arrived in June) Your Eldest who is 11 will probably go back into year 6 My eldest daughter was 11 (12 in Sept) when we arrived and went back into year 6 and completed the year. I could have pushed her into year 7 but lots of schools have mixed year groups, say a 5/6 class or a year 3/4. I thought redoing half the year would give her some confidence with the work and allow her to make some friends just in case they mixed classes at the end of the year. My middle one was 9 (10 in the Sept) and went into a 3/4 mixed class. The hardest thing for me to get round was that hey were always the eldest in the Uk being Sept birthdays and here they are amongst the youngest.
Basically with all the school age kids you'll be looking at redoing the year they will be leaving in the UK. As most schools here are Reception t Year 7 they will all be on one site.
Once you have decided which school you like, have a chat with the Principal as they are very open to moving te kids up a year dependant on their work and most kids tat come from the UK are advanced as the Curriculum there is much stricter. When we arrived Natalie was doing work in year 4 she'd been doing in year 2, I think it was something to do with telling the time but you just have to forget all that went before you and concentrate on whats happening now. "In England" is a phrase you have to put away and not use.
Now Matthew will have to go back into Kindy as they don't take them to school here until they are 5. Unlike the UK where there is only 1 intake a year, the kids come to school all through the year. This means Matthew will probably only do 2 terms of Reception and then go to year 1.
As the Uk there is 4 sessions of Kindy a week of 2 1/2 hours. You can either do morning or afternoons or if you are working, some Kindys do 2 whole days with a lunch break.
you have to pay for Kindy here, its roughly $55 - $65 a Term (10 weeks). There is no Government funding but you do get discounts on Childcare depending on the family income. some are actually on the school site which makes life easier and there are others in separate locations.
you can access all sorts of areas from that Education website and I think there is an area to cotact them by email, I'm sure they can help you.