Took our twins on the LGW-UVF run a few years back now, when they were not quite yet 4 - no probs at all - it's all about being prepared (take lots for them to do, don't expect any type of IFE to keep them completely occupied) and being relaxed (meaning don't get stressed if they do). One more thing, if they want to sleep (and you can't move as a result!), let them, no matter what time the real or body-clock says it is!
That was our first long (-ish) haul with them, done 3 or 4 more transatlantics since and now they know what to expect, how to behave, it gets easier each time. We're happy enough about it all to be taking them to New Zealand next year (by which time they will be just past 9th birthdays).
Of course, it also helps a lot if your kids get on
PS - if you're on an Odyssey-equipped aircraft, the "Millionaire" game is quite addictive, if you're into that kind of thing!