35" seems tall for a pack n play. My older son did not sleep comfortably in a pack n play at that height and even a little smaller. My younger son is currently 31" and he doesn't have much more room to go before he is too big. The pack n play and the the cribs Disney provide are both smaller than a full size crib. We got a real crib last time we stayed at the GF and it definitely wasn't as big as our cribs at home. I'm not sure how far away your trip is, but I wouldn't count on your 3.5 year old being comfortable in the pack n play, even if she is small for her age. The Murphy bed is a consideration, but you can't use bedrails on that and it is somewhat narrow. Granted, it isn't a long fall to the ground. I would put the 3.5 yr old in the middle of the sofa bed and the 6 year old on the Murphy bed. This is probably what I will do with my boys on our next trip (they are 4 and 2).