Typically, unless your child is going to use the bassinet, then you probably won't get the bulkhead row. Reservations will keep that row available for someone who might actually need it.

For HKG-KUL, given how often CX offers discounted fares on that route, there is a good chance that loads aren't great and there will be empty seats on the flight. If you have any status with Oneworld, you can ask the check-in agent whether they can block off a seat for you and your (lap)child.

Have a good trip