Hi there,
I've just flown to Thailand and back with my nine month old - I was dreading it at first but it went a lot smoother than I thought! I would try reserving the bulkhead seats if you haven't done so already as they at least give you a little extra leg room and there will at least be no one in front of you putting their seats down.
We took the ready made cartons of milk on board through Heathrow - they only made me try one of the cartons and once it was opened, I was allowed to pour it into a bottle so that it wouldn't be wasted. WE also took my daughters favourite food on board as well in case the infant meals weren't great.
Ours was a day / half night flight so a bit more entertaining needed but there happened to be quite a few small children on our flight so we took our daughter down to the back of the aircraft where there was quite a bit of space and she played with her toys down there. Also took one hour shifts with my husband so one of us could rest whilst the other was on "duty"! This worked really well in passing the time away.
Before we knew it we had done twelve hours! And its all worth it once you get there.
Oh before I forget, my daughter didn't sleep long - only three hours but it meant she slept amazingly once we got there and straight onto Thai time so no jet lag!! So don't panic if they don't sleep as much as they would at home
Have an amazing trip!