The 4 year old should definitely still be in a car seat for safety. You should bring their seats on the plane, especially for the 2 year old. You wont know if rented/borrowed seats will be the right size, clean, expired, ever been in a wreck, etc. best to bring their own seats that they are familiar with and you know the history of. If their seats are too big for travel, their are good travel options out there. I believe the Cosco Scenera Next is the one frequently recommended for younger children as an inexpensive lighter weight seat for travel (under $40). Depending on the 4 year olds size they might be able to get away with the bubble bum inflatable booster seat for $30. My son turns 5 the week after we cruise and he is huge-nearly 70 lbs and almost 4 feet tall. Hes normally harnessed and were driving to port with his harnessed seat but for port we bought the bubble bum