The Benadryl thing someone mentioned isn't good. I've noticed that ads for Benadryl lately say things like "Use only for the symptoms listed" etc. because the makers know parents are using it to knock kids out and don't want it used that way. It can actually have the opposite effect! I gave it to my daughter once for allergies when she was a toddler, and she was totally wired by it and I know other parents who say it winds up their kids rather than making them sleepy. I wouldn't use any meds solely to make a child sleepy for my own convenience on a trip.
We travel every year with my daughter (who is now 10) overseas. You're right, the sleep timing is tough for small ones and even for bigger kids. Usually they will end up falling asleep on a train, plane or in a car out of sheer need, even if they are "picky sleepers" who wont' usually fall asleep just anywhere (my daughter was the same). But sometimes you just have to be ready for a first day of bad temper and tears until their bodies adjust. For a trip as short as two days, frankly, if it were an event I really wanted to/needed to attend like the wedding of someone close to me, I would go and my husband would have stayed home with the little one, or vice versa. That's too short a trip for a small kid to adjust and get on a schedule there, and when you returned home you'd have had a day or more of real crankiness and sleep disturbance. A longer trip is actually easier since they can adjust in between travel times.