When our son was younger, we always selected early seating. Sometimes waiting until 8 or 8:30 is just to long for kids to wait. Early seating will allow you and your kids to have dinner together and then head your own ways with plenty of time left in the evening. It isn't unusual for late seating to end after 10:00pm, which is late for some kids (unless they are in their late teens, say 17 to 19). It really depends on the kids ages and when you folks dine at home.
You can always request a change with the Maitre 'd once you are on the ship.
Just my opinion.