Swimmy Demo Image (Squeak 3.8 image and change file): Swimmy-Demo.zip
Please download Squeak 3.8 VM and run the image on that.
Paper: Koji Yokokawa "Swimmy: A Framework of Multi-Agent Instruction System for Children" in IEEE the Third International Conference on Creating, Connecting and Collaborating through Computing (IEEE C5'05), pp. 27-32. (full text PDF)