It will be fine!!We've taken our children to the USA every year from when they were 12 months old and I honestly think they cope better than we do!! My hints would be..... Lots and lots of snacks given out at regular intervals during the flight (prob not the healthiest 10 hours of their lives but hey-ho!!! ;0))
Also, we wrapped up lots of individual little toys (toy cars, crayons, stickers, little men etc) and they got so excited about choosing something from the bag and played with it for ages then when they got fed up, they got to choose something else.
In addition, we found a personal DVD player to be worth it's weight in gold so they could watch all their favourite cartoons (would second the iPad idea for sure, but DVD player is a cheaper alternative).
We have always found that our children slept for at least 2-3 hours minimum (last year our 3 year old slept the whole way home from LAX on a night flight!!)
And lastly, make a big deal out of the take off, drinks service, food service, going to the bathroom, taking a wee walk around the cabin etc etc.... It's all so exciting for children that this can easily amuse them for a while!!
Good luck and happy holidays!! :0)