We are returning for our 9th WDW trip on the AT
We have traveled coach in upper level. the sway isn't too bad at times. The lower level bathroom doors don't get closed after their use and can bang all night long. We had that lovely experience while traveling sleeper lower level.
If the train is not full you can spread out. they tend to keep families with kids together.
I would get a ticket for the youngest. Our youngest to travel was 14 months old. We had a seat for him. DH took the foot rest of the seat and pulled it all the way up to create a wall. We took a pillow from home and DS slept on the seat cushion the whole night. We did alternate seating next to him during the night. There was no danger of him falling off the seat bc of the foot rest being in an upright position. Last year DS4 (now) slept on the footreast and seat botton the entire night. I raised the footrest to be level with the seat bottom and he slept pretty much the full night. When he was 2 that was a hard trip to hold him and sleep at the same time.
It is 18 hours on the train but the majority of it is during the night. Most kids are lulled to sleep by the rocking of the train. Make sure to bring her favority blanket and soothing toy. Pack extras in diapers and snacks/drinks in the event of a delay. I use to wear DS when traveling btwn trains bc it does rock fairly well. It allowed me to use both of my hands to hold on (esp if it rains).
HOpe this helps. Let me know if you have any other ?"s