My daughter was 5 (almost 6) on our first cruise (Bahamas) and we did second seating, she was 7 for our second cruise (Caribbean) and again, we had second seating and then for our most recent cruise (age 8) in Alaska we did early dining and I will say on the main dining - never again!
As you can see, opinions vary and you'll figure out what works well for your family, but for our family, early dining didn't work for a number of reasons. First, my DD and DH sleep in most mornings and so falling asleep early wasn't ever a problem. We are usually sleeping in and staying up late anyway. Our main issues with early dining were as follows:
1. We had to be somewhere at 5:45 (as opposed to 6:15).
2. Our time between dinner and the show was cut short because the we were always headed to the theatre way way early to get a good seat. With late dining, we never had issues with seats. We waltzed in 10 minutes prior to showtime and had the pick of seats. We found out the first time we needed to get there 30 minutes early to get a seat together. Thus, we had less time.
3. With main dining, we were done with the show at 9:30. With second seating dinner, we were generally done with dinner around 9:30-9:45 so we're talking about maybe a 15 minute difference in what time DD went to bed. That's relatively a small amount of time, especially considering we lost 30 minutes in the afternoon and another 20 minutes or so between dinner and the show. So to get to bed 15 minutes earlier, we "lost" upwards of 45 minutes in other parts of our day.
4. We had to get up early on disembarkation day. Then after breakfast we had to wait around over 30 minutes for our character to get called to disembark. With late dining, by the time we were done with our breakfast (after sleeping later) we could just walk right off the ship because all the baggage tags had already been called.
5. We always seemed to want a late afternoon snack and on several occasions, we totally ruined our dinner. There's always something to eat on board and when we didn't snack late afternoon, it felt like a bummer! But we didn't want to ruin dinner. We also like popcorn during the evening show and we're too full to have some after dinner.
Add to all this that DH has restless legs and they seem to hit him every night we were in the theatre. They never bothered him during our dinners though around the same time when we had second seating. Maybe it's because he feels more comfortable to shift around? I dunno, but he had to leave the theatre every night to walk around the ship instead.

He missed all the shows.
The funny thing was that somewhere along the way, I got a bee in my bonnet that I HAD to have main dining for our most recent cruise to Alaska and then we all ended up not liking it! We're from central time zone and my daughter doesn't adjust very well across time zones so I thought that eating at the equivalent of 11:15 PM would be awful, but as it happened, it would have been much better to stick with second seating.
So, we're officially a second seating family from now on, no matter the cruise or time zone!