As has been stated the early/main seating is usually best for kids, but it depends on your eating habits and preferences.
You can request a change any time up until sailing. If they can't confirm the request they will waitlist the request. You can still request the change once onboard as well.
Cruise Fanatic also mentioned the Open Seating option. It is now available on the Carnival Pride as of last week. It has been my experience in the past that the early/main seating is usually more in demand than the late.
Which ever you decide, don't forget to try the pumpkin soup and chocolate melting cake.