As an alternative to a DVD player, check the entertainment offerings for the airlines you are considering. Especially on long haul flights, there usually is a kids channel or two. Perhaps your nieces could learn Japanese on the way over?

Another idea is if you take a laptop or ipod for yourself, load them up with movies, at least for the 4 year old. I doubt the 20 month old will watch much or that you'll want to hold a player for her until the batteries run out.