We just got back a week ago from our cruise, and we did the second seating dinner (show at 6:15, dinner at 8:15). The nice part was that it was easier for the kids to stay awake during the shows. It was a pretty late dinner, though.
Our kids are 6, 4 and 2. What we ended up doing that worked well for us was to eat a snack in the late afternoon, get dressed for dinner before the show, go to the show, then after the show DH would take the 2 year old to the buffet and feed him dinner while I did something else with the older two kids, then we would take the 2 year old to the nursery when we went to dinner.
With the second seating they have "dine and play", where the kids come to dinner with you, and they get the kids' food out pretty quickly, then after an hour, some counselors from the kids club would come to the restaurant, and pick up any kids that wanted to go to the club at that time, and take them there. My older kids did that every night, except the night we ate at Animator's (so they could see the color show and Mickey), and the Pirate night, so they could participate in the pirate party. They LOVED the kids club, so they were happy to be able to do that, and it was nice for the adults to be able to finish dinner without having to rush due to bored kids. We usually finished dinner around 9:45 or 10, and then picked up all the kids, and went to bed.
It did seem like most families with young kids went with the earlier seating, but the second seating worked ok for us.