Many years ago I read this book. I also saw the film. I was sure that at the end of the book the 'cuckoos' removed themselves from the neighbourhood by the simple (to them) expedient of making their immediate environment invisible to the rest of the village because it was a fraction of a second in advance of the rest of the world, i.e. it hadn't 'happened' yet to everyone else, but they themselves could see out because it 'had' happened to them. This didn't appear in the film - did it occur in the book or am I thinking of another one altogether?
I think you might be thinking of something else. As far as I remember the book ended with one of the adults, I think it might have been the doctor, blowing up the house that all the kids lived in and killing all of them, including himself.