There are reasonably priced flights from several UK airports to Australia's 10 main international airports, including Sydney, Adelaide, Melbourne, Perth, Brisbane, Darwin, Cairns and Canberra. Direct flights from London take 21–25hrs. A host of low-cost airlines operate internally for those planning to explore more than one area (travelling from Perth to Sydney is like going from London to Cairo, while Cairns and the Great Barrier Reef are in the far north and Uluru/Ayers Rock is in the middle, so even covering the highlights requires a lot of moving around).
Take a look at our flying with kids tips page for advice on handling such a long flight and the inevitable jet-lag. Seat-back TVs with kid channels, fun packs, Sky Nannies and so on make life easier. You might also turn the stopover into a mini family holiday in itself by breaking the journery for a night or two in the Middle East, Hong Kong, Bangkok, Singapore or even LA.
See also our feature on exploring the Outback by motorhome. There also also some amazing – and child-friendly – great train journeys in Australia.