I consider how tired kids are during the day. If they're grumpy and have difficulty participating in daytime activities, I consider going to bed earlier. Some kids need more sleep than others. If they are consistently grouchy from lack of sleep, I'd put them to bed earlier.
I do think that bedtime should be close to the same time every night except for special events. Camping at the beach qualifies as a special event. My grandkids do better keeping some sort of routine as far as bedtime and meals are concerned. If your kids are more flexible, i.e. they may do alright with a more flexible bed time. At the age of your kids, my grandchildren weren't able to adjust with differing bedtimes. If they got to bed late, they still woke up early and couldn't take naps and we're grouchy the next day. So with a later bedtime, I planned the next day to be a quiet day without structured activity.
Bedtime at 8:30-9:00 is certainly a reasonable bed time. My 2 an 3 yo grandchildren's regular bedtime is that. Their bedtime is based on when they consistently get tired. They rarely go to bed later than that because by then they fight with each other, they cry easily, and are difficult. Their bedroom has black out curtains.
I suggest bedtime should depend on the amount of time the child needs to enjoy the next day. It's OK to be lethargic on some days but not everyday. I suggest that if they are cranky and difficult they need more sleep or need more food.
My daughter and her husband do not have regular meal times. The kids are frequently cranky when I come to watch them. I've found that feeding them improves their demeanor.