What comes up must come down.
If a missile defense system is activited and shoot downs a missile, then whoever is beneath this will see fallout.
If you look at this map, then you will see that the shortest route for a Chinese missile would be straight north over Russia and then south over Canada which would be a lot shorter than going over the Pacific Ocean. Therefore, it is actually possible that if the US would shut down a Chinese missile, it would do so over Canada.
I do find it very unlikely though that any sane Canadian Prime Minister would actually agree to that or even agree to meet with the US President to discuss such a thing.
