Hi, many do bring their own car seats. If your child is under a specific age or weight it is the law they be in a car seat. Sorry I dont know the exact weight or age for you.
Some airlines require it too if the child has their own seat.
Just to make it easier for traveling maybe you could ask if the car rental place rents car seats?
If not you will probably need to bring your own.
I do believe it is a good idea. Since you wont need it on the ship, maybe the Radisson has a place to store it for the cruise?
Or if your returning the car at the post and then renting again after the cruise, maybe they will le tyou store the seat there for your cruise?
It cant hurt to ask.
Good luck,
Kathleen