It is the continuation of the scene that begins at minute 1:05. We discover that it is not only Leo who is spying on his parents but also Ale (his sister). Shortly after their parents start having sex Leo covers his sister's eyes.
Wendy and Sandy are at band practice. Wendy is sitting center row, playing a trombone. Sandy is playing a triangle, sitting directly behind her, slightly elevated. He is intensely peeking downward. We get a brief shot from his POV, staring at Wendy's butt crack.