I have 3 small boys, and travel back to the US.
If you travel every week and give them drugs, then they can be dependent on it. But if it's a long trip, and it's 1-2 times a year, use the drugs.
My sons' can't use benadryl. They hyper up. I found that out of a "long" trip from NYC to LA.
As far as "ethics" are concerned, it's very difficult to change your clock, be stuck in a plane for hours and not get agitated or cranky. We take drugs to help us, tylenol, Melatonin, or whatever, what is wrong with helping the little ones make their trip a little less stressful? Sometimes I have a glass of wine with dinner and it helps me relax too! Am I now dependent on it?
The best place to start is with a Doc. Tell them your impending trip and get their advice. After that, try the medicine a few days/weeks before you travel just in case they get hyper.
good luck!