The 8th episode "Jak jsem se poprve zamiloval" (4th on DVD): Short scene near the beginning, rear nudity of Stepan (Marián Beník, 12) and full nudity of Matej (Pavel Zedníček, 10) as they are showered in front of the house (partially covered by Czech episode title, so possibly not seen in foreign versions if title sequence was changed).
Sarah takes advantage of the oppertunity to find out as much as she can about sex. In one scene, she convinces Jimmy to show her his penis. Jimmy seen from the side rear (butt not visible) to show Sara's reaction.