I have just flown 11 hour flight on Virgin and on outbound flight, on my own with 2 DS's (4,6) we had economy seats and I would say the opposite to inthesleeplessnightgarden. Both my DD's couldn't get comfortable in economy even with the arm rests up and seat back. They were trying to curl up in the seats and just couldn't get comfortable.
Also saw some parents who had the bulkhead further down the plane who had both put their kids on the floor and were having an argument with the air hostess who made them pick the kids up and had to have them on their laps the whole way, and they weren't small, maybe 2 ish at a guess. I did feel very sorry for them as the flight was full.
On way back we upgraded to Premium Economy (using DH's frequent flyger points) had bulk head, me with DD2 and then DH with DD1 on other side (window seat) and it was much more comfortable for both of them, with the seat back. As the seat is wider they were able to curl up and get more comfortable. The hostess even bought out foot cushions to make us more comfortable. BTW upgrade using points was the same for me and the kids, aren't they mean that they don't make it less for kids?
I told DH next time we do a longhaul flight we are going to save up for premium economy as it is so worth it. I have done economy flights all over the world since kids were babies and never again. Wish I had enough to pay for business class for us all but that really is wishful thinking. 
And another tip take something to entertain them in landing and takeoff (we had to circle a few times on return journey as it was so foggy) when you have to turn off electronic devices e.g. iPAD. I had some colouring in books and stickers to keep them occupied. Worked a treat.
Sounds like a fab holiday and every envious.