Whilst relevance doesn't always lead to motivation, I think this is likely to be the case here. I would first of all say, you need to see how they feel about the move to Australia. If they're excited, then some realistic materials about Australia could help motivate them. And even if they're not particularly thrilled with the idea of moving there, the fact is that they will need to be making new friends etc. In that case, IÄd focus on their hobbies and interests - if they're interested in sports or music, for example, being able to talk about in English will help them communicate to make friends with other kids who are interested in the same things. And talking about their own interests should be fairly motivating, too. Otherwise you could think up a kind of mini reward scheme or set them goals to reach - I can imagine "learning to speak English" is a fairly daunting task when faced in one chunk, so break it down for them!

Hope that helps - good luck! CMF