I keep trying to let them have more freedom about bedtimes, but sometimes it "works", most of the time it's better to get them in bed.
On gorgeous summer evenings, the rule is they have to play outside if they want to be up past bedtime. I tell them if all goes well, I might just "forget" the time. That works once in a while, not as often as I'd like. Other times of the year, if they have a great game going indoors, I'll say the same.
Solstice times really are hard on sleep. Summer solstice, it is light from 4am to 10pm where we live. Winter, it is dark from 4:30pm to 7:30am. Too much light keeps them awake; the dark is disorienting. Equinox time they sleep the best.
My oldest just negotiated a later bedtime in exchange for some time to "play the toys away". One day, one day, when I don't have to be ON all the time they are awake I will be more accommodating.