My wife and I took our daughter to Europe this past Summer. She was ~3 1/2 at the time and did extremely well on the flights (Seattle to Atlanta, Atlanta to Barcelona, Malaga to Brussels, Amsterdam to Seattle) and also had a lot of fun everywhere she went. Flights get better once the kid has their own seat, and make sure to have lots of stuff for them to do.
We left her home with my parents when we went to Japan right after she had turned 2, though.