We've traveled a bunch with our now almost 4 year old, including a couple of trans-Pacs.

- Transpac at 4 months (CX F) wasn't bad at all. 2 F seats and a lap child, he didn't sleep a ton, but he was happy and quiet. Great seats, great service, etc. On the way back, we had the F section of a 747 to ourselves - perfect. It also wasn't his first flight.

- At 3 1/2 years (CX J), it was good. He had his own seat, and slept a lot and otherwise entertained himself with the AVOD, iPAD, etc.

However, the time between the ages of 1 1/2 and 2 were by far the hardest for us from a travel perspective. Even short hauls domestically were tough. Old enough to want to move, too young to be entertained for long. Unless your toddler is one who can sleep anywhere/anytime, a transPac would be really difficult, IMO.