the ending
what was up with some of the kids leaving and them being like "can we come with you?" and they were like "no we will come back for you"
didn't get that.
I don't think there was anything to "get" other than that the children are about to spread their disease/murder spree onto the mainland. It was somewhat affecting, I suppose, to see how sweetly the children treated each other (speaking nicely, assuring each other they'll come back to take them to the mainland) given how they treated the adults.
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Gentlemen, you can't fight in here! This is the War Room!"--Pres. Merkin Muffley