I think the OP is asking whether or not Cathay would bump someone else off their assigned seat in order to make room for one more passenger travelling with the OP.

In the OP situation, I would try to find another row where you can sit together. If your flight is more than a few days out, it should not be a big problem.

Cathay probably will refuse to bump off the other passenger from their seat unless you are a DM or the flight is overbooked and the other person has some elite status, if you ask at check in to solve your problem, they may decide to bump that person to Business instead of asking them to move to another Economy seat.

Finally, if you have a comparable aisle/window seat to offer, you could nicely ask the person in that seat to swap to help accommodate your family. As long as you're offering a similar seat (aisle for aisle in same cabin section, for example), many people would agree to your request.