Looking through a crowd of people, we see a boy (6) and a girl (8) full frontal and a couple minutes later as the crowd walks away, we see them also walking away with the crowd.
The film is set in a world where clothes have been banned, so all characters, adult or child, are naked at all times. Boys play on the beach, views are distant.
In another later scene, boys (7-9) skinny dipping from another part of the water rush over to see the older boys mocking a girl. Some girls (8-9) are topless.
The 12th episode "A Story of Ferryboat" , dated 11.04.1981: Naked boy running around with his sisters for about 5 minutes starting, at 0:02:01-0:06:43.