I have a German ex and the
kids grew up in a dual language house. I'd go to
work and the house was full German till I got back. German cartoons, German story
books, etc. It took some effort/discipline to keep the house in a different language but was worth it. One thing that really helped was to keep making them proud of knowing more than one way to talk. Of course, the rest of their friends spoke English and it was on the TV and in
school, so they had no problems with it. Then the
kids picked up Spanish in high
school and from friends. Out in public was fun because most Americans are stuck in English so we could talk privately by just switching.
My first two thoughts are to start them as soon as possible and the 2nd language almost doesn't matter, the point is for them to get used to being able to switch. After the first couple of languages, picking up more gets more interesting and less difficult.
The trip is in the future, they are Canadian citizens, and you live in
Canada for the foreseeable future, so the obvious choice to me is French. There were so many French colonies that it'll help wherever you go. Plus, picking up Spanish words are pretty easy if you already know French. Since you are already fluent in one of the most common Asian languages, make a
game out of teaching those words at home, too.
Their ability to pick up and learn at this young age while you have complete control has no limits like we all have as we get older.
Most of all - Have fun with them in this!