Only 10% of cruisers get seasick under normal conditions. Like PP said, they are probably the same people that are susceptible to get motion sickness on land, in cars or theme park ride.
There are some zones and weather conditions that will make the ship move more than usual. The captain tries to limit those situations by going around bad weather or going slower but it is not always possible. For those occasions, always bring medication that you have tried out before on land, like Bonine, and use it the second you feel you might get sick.
I truly believe that having anxiety about motion sickness is a major risk factor. If you can, don’t talk about it too much in front of the kids, outside of simply mentioning to talk to you if they feel queasy.
I also believe that stateroom location has little impact on you getting sick or not. If the ship hits bad weather, you will feel the extra movement all over the ship, at any time of the day. The logic is that a central and low location will result in you feeling less movement if you are in your room, but the downside of staterooms in that location is that they are oceanview and inside staterooms, without access to a balcony.