i'm always in the minority on this issue, but thought i'd weigh in anyway. my four year old is a seasoned traveler and has been doing at least one long-haul trip to asia every year since he was a couple of months old. we've discovered that non stops are just not an option for us- at least not on the US to asia routes we typically travel. we NEED that stop in FRA or BRU or wherever!
the only non-stop we've done with my son is the old TG jfk-bkk 18 hour flight in 2008, just before they did away with that route. it was pure, sheer, unmitigated hell. ok so he WAS 8 months old and teething at the time, which obviously contributed to his irritability and restlessness, but still...my husband and i spent what felt like the most part of that flight walking up and down the cabin, holding him in our arms and trying to soothe him. he just wouldnt settle and sleep beyond a certain point, though he did have his car seat in his own seat. cant imagine how traumatic the flight wouldve been had he been a lapchild! as it is, we couldnt wait to get off that plane and let him crawl around unrestrained.
subsequently we've found that what works best for our particular child-yes of course every child is different- is a stop of a couple of hours where he's able to stretch his legs and burn off energy, enjoy the comforts of a lounge if we're flying biz or just wander around the gates and stores..we've tried everything to entertain him on long flights, and have discovered that no matter how entranced he is initially by new toys, books, an ipod, ipad or IES, after more than 4 waking hours on a flight he's DONE!! (needless to say by then we're frazzled wrecks) so i cant imagine we'd be able to cope on a non stop to asia, which tend to typically run 15+ hours
a straight up transatlantic to europe would be worth considering though and i can see how it could be painless, especially the night flight out..good luck!