JJsmama,
We have the same dilemma, a two and a half and four and a half year old boys with a late seating on our seven day cruise. We've done this before (almost two years ago), and found that it worked out ok. We usually took care of feeding the boys at their usual time at a hamburger/pizza/lunch type of bufet, and then found that dinner worked out ok. We only had to worry about feeding ourselves a good meal and the boys having a "snack" while we ate. Sometimes we let the older one go to Fun Factory (kids club), sometimes we got babysitting for the little guy after dinner (it was a 14 night), and sometimes they'd nap at dinner (we put the big guy under the table with his head on a napkin, and the little guy would nap in a sling). The key was to feed them when they were used to eating, and it made an enjoyable evening for us. We'd optimally choose the early seating this time, but it wasn't available (last time we chose the late seating, and it worked out b/c we fed them at their regular time).
PS our kids have an 8:00 bedtime at home too, and we live on the East Coast.