We have done three cruises: two with second seating (late) and one with main dining (early). My daughter was just shy of 6 on our first Disney cruise and we had second seating and we were very happy with it. When she was 7 on our second cruise, we did second seating again, but for our third cruise, I decided to try early dining (she was 8) and we found it is not for us. (We wait listed for main when I suddenly decided we should try it and got it a few months before sailing.)
We found that we somehow ended up with a lot less time on our hands. We felt like we had to race to dinner at 5:45 (instead of the first show, which is 6:15) so we had to be ready 30 minutes earlier. Then, we found on several occasions that we weren't very hungry because we had inadvertently snacked far too late (and more heavily than we intended). Then we felt like we had less time between dinner and the show, mostly because, to our surprise, the second show was much more heavily populated than the first and we had to get there 30 minutes early just to get seats together (with second dining, we were able to roll into our show 5-10 minutes before show time and get great seats).
Also, dine and play is great (the waiters fast track the kids through dinner so that councilors can come pick them up and take them to the club). DH and I were able to enjoy a kid-free meal a couple of evenings.
My daughter is in bed by 8 PM every night at home, lights out at 8:30 and she's asleep by 9. On a cruise, she definitely has a completely different schedule. She sleeps in in the mornings and if we have a really late night activity we want to do (Pluto's Pajama Party is one of her favorites and it tends to be late) then we try to get a nap in mid-day. Even long after dropping her naps, she will nap on a cruise (and we do this at WDW too) to take advantage of late night activities.
So, if you don't get early dining (though, at this stage, you likely will), you never know - it might be a win!