As best I can tell here is the story of this street and the sign.  In 1217, Enrique I gets killed, supposedly by a tile falling off of the roof of his castle.  Notice the tile in the top left of the street sign.  The noblemen hid the truth about the death of the King.  When his sister finds out about it she become Queen, but she almost immediately renounces the throne in favor of her son who becomes Ferdinand III.

The two youths shown in the picture were the ones who they said were working on the roof when the accident happened.  The two youths had escaped to Madrid and were caught on this street.  They were quickly executed before anyone could discover how the King may have really died.

The street was named after them.  "The street of the Youths".