One of my school friends emigrated with his parents and four siblings - he was 14, his brother 15 and the others were 9, 10 and 11.
Even though there was no NRP involved in the decision, the 15 year old wasn't keen on the idea but was overruled, obviously.
He went to school, got a part time job and one month after his 18th birthday, got on a flight and came back to the UK, sleeping on various schoolfriends' floors until he got a job and a room of his own.
One of the younger siblings came back when she was 18 as well and recently, my schoolfriend came here with his wife and two children, with no intention of ever returning. So, despite there being no extra complications, the intent of a 'better life' hasn't really worked well, as the parents are over there with two out of their five children (and I've no idea of their ongoing relationship, it might be brilliant, it might not).
Not everybody likes Austalia - or moving.
Another friend disappeared over the summer holiday aged 15 because he went home on the last day of term and was informed they were emigrating at the weekend. The only thing that keeps him in New Zealand is that he now has two daughters with his ex (they apparently split up because she didn't want to come to the UK) who he feels he can't leave.
I'm not saying don't go, but I am saying you should consider what the consequences would be if either of the DC decide this isn't what they want - do you let one stay here, or risk finding out at 18 that they've gone and aren't coming back, or have them permanently feeling that they aren't 'home' but are trapped?