In 2005 when we did our reccie, we worried about the travelling with our then very active 3 year old daughter. In the end we decided to travel with short breaks at various airports along the way - so went from Birmingham to Frankfurt, then hung around for 2 hours there, then onto Singapore (the long stretch of the journey) where we had a 4 hour wait and then onto Perth.
Luckily for us our little one was past that exhausting 'up and about and all over the place' stage but still it was a concern. The stops at each airport gave her enough time to charge about and knacker herself out for the next leg of the journey...and of course the airline tv network helped too. At Frankfurt airport there was super little train play park, really well thought out and ideal for weary parents to stand by and watch!
Good luck and try not to worry...one thing I would recommend however is to take doses of 'Rescue Remedy' on the journey, it works wonders for the anxiety and stressful side of travelling and if given to little ones can help them with the jet lag etc You can purchase it from Holland & Barrett stores or Boots, look for the homeopathic and herbal remedies section, in a very distinctive yellow box.
Bye for now and happy travelling...