My view is that you might want to consider how your child "resets" to the new time zone on arrival and not just focus on the journey.

Haven't done the crossing dateline but have travelled from East to West alone and with a child a couple of times.

For our next E-W journey, Beijing to London, my plan is to thoroughly exhaust my daughter, day of sight seeing and then a swimming pool session before we set off to the airport for a 1:30am departure. Prediction is that she'll be asleep by 11:30pm and will awake somewhere close to England. Knowing her steely determination to experience "fun" she may even stay awake for boarding.
Every kid is different. My problem is that my own child LOVES flying, doesn't matter if the lights are dim and everyone is asleep, an aircraft cabin with kind crew members, packed with hundreds of people is the most exciting, stimulating environment imaginable for her.
Purely selfish on my part, but a co-operative & benign zombie child is so much easier for me to deal with than a preternaturally alert and active little girl in surroundings that are beyond fascinating to her.

Best part of our E-W overnight flight is that she'll be waking up in the morning which will make her internal clock adjust easily to the new time zone. It's a completely different story when travelling in the opposite direction