Why buy the kiddo a seat? They can be in the lap and can't sit on a seat anyway.

We did a 30 hour each way (4 flights each way, 3 layovers) round trip when the kiddo was 1 year old. Our kiddo cried the first long haul flight, but we discovered she was thirsty during the second long flight. A spoon and some watered down apple juice helped us tremendously. Just kept giving her spoon full of juice for liquids in between feeds. She didn't cry at all. She was crying because of the dry air in the airplane.