No way, I wouldn't do that to my child. I already flew to Alaska with my son who was at 8 months old at the time... he slept most of the way on his own, and I flew to Washington with my son who was 14 months old and step daughter who was 12 at the time by myself, and it went good most of the part but at the end, he was throwing fit because his sleep was disturbed. I flew to Florida with my son who was two years old, and that was 6 hours flight.... it was hard to keep him entrained most of the time, to keep him in his seat but I managed and thankfully he slept the last two hours of the trip to Florida. On the way back home from Florida, which was 8 hours trip, he slept most of the way there but still, I managed to keep him seated and entertained just with DVD player and DVDs.
I guess what I'm trying to say is that You don't need to put kids to sleep with drugs... you just have to give them your attention and keep them entertained. With Young kids, I would take Red Eye Flight... which I did most of the time with my son, as the kids would most likely sleep most of the flight if it was Red Eye on their own.
However, if I was to travel to Europe which is likely to happen in the future, I would most likely give my son like allergy medication that would make it drownsy, and perhaps he'll fall asleep due to drownsy.