Hard to tell what you thought was the outstanding rectangle, the tones weren't the sharpest, and the comp could have used some extra thought. Maybe, a different cropping and some exposure adjustments would have helped. However, it was your title that took this right out of the Challenge's concept for me.
Unique take on the challenge and I like it .. a lot! Nothing to suggest as an improvement .. it's fun looking for the rectangles and I consider the chain in the child's hand to be one as well.. you caught the expression children of this age show when they are very involved with what they are doing. Great shot.