HI Florenceuk, Sorry for the late reply -- I've not been on mumsnet for a few days SHOCK SHOCK
Anyway, back to travelling alone with two kids. I think the most important thing is your own mental state. If you can get into a mindset of not expecting to do anything for you, and concentrating entirely on your kids, it helps a lot. So don't even look at the movie selection (apart from what is on the kids channel) and take it as a bonus if you get to eat your own meal in any degree of peace.
As for things to entertain kids, I have found the following the most useful:
play dough
sticker books
story books (that your elder child can also scribble all over, my elder one liked those books where you can point out things -- something like 1001 things to spot in the sea, its an Osbourne publication)
treaty snacks, like mini mars bars for bribery
pyjamas to change into at "bed time," so you can make a bit of a division into day and night and hopefully get your elder one off to sleep
I also gave my kids a sedative on the flight (rhinathiol promethazine, its a cough mixture) which helped them to get off to sleep. But don't do what I did on my most recent flight back from London, where I bought some cough mixture at the airport (Actifed, I think..) anyway, although safe for children, it has a horrid taste and I couldn't get either of them to drink it...
What else? Nappy changing will be a bore. You will probably have to take both kids with you to the loo. DItto when you want to go to the loo yourself. But at least its a change of scene for a moment or two. ANd also if your elder child is out of nappies, I would put him/her back in nappies for the flight: If you are feeding the baby, you won't be able to jump up and do a loo trip straight away.
Finally, as I mentioned before, I take a very lightweight pushchair (maclaren volo). I've never had any trouble taking that on board. ANd it is very useful to have it when you are disembarking. Infact, I would make a bit of a fuss about this if you have to, as Heathrow has got all strict recently about not letting you collect your stroller at the aircraft (if they put it into the hold) and the last thing you want to do is traipse though all those corridors with a tired toddler and a baby in your arms -- assume you'll be taking a baby bjorn or similar baby carrier for the little one?
Oh and one last thing, as well as spare clothes for the kids, take a spare t-shirt for you. Its quite likely you'll get something horrid down your front....
And one last, last thing. Its only 24 hours of your life. Its not that bad, and putting up with the journey means you can go to New Zealand, which is infinitely more fun than staying in England in February.
HTH, Lorien