I'm doing Brisbane-KL-London-Cork with a 5yr old and 18 month old soon! I've done it 3 times before when my daughter was 8months, 2yrs and 3 yrs. First of all it is never as bad as you think it will be, as the previous posters have said, bring lots of toys, little things from the $2 shop that you can just toss out when the trip is over, bring a new one out each time they get bored of the previous one. I brought lots with me and honestly I only ended up needing a few. Young children tend to sleep a lot on planes, you will be surprised. Good luck and try and enjoy the trip!