The period when you're looking to fly your nieces to Australia is high season for anyone leaving Europe and therefore prices are high, but there are still some reasonable deals around. For example I just found flights on Etihad flying London-Sydney at the beginning of July and back in late July for a total price of ??866, The outward flight takes 28?? hours, with two stops, while the return is a little over 25 hours.
I originally found this on the metasearch engine momondo.com, but this is the price obtained from the Etihad website. Flight search engines are a great place to start looking for cheap flights. The price on Momondo was about $100 cheaper than that offered by Etihad but there's an added layer of comfort and quite often extra privileges if you buy from the airline itself.