Children under 2 years old, or who have not been potty trained for more than 6 months must wear approved swim diapers in the pool. Approved swim diapers do NOT include "Little Swimmers" or similar diapers. Plastic pants or reusable swim diapers are acceptable. These can be purchased at the Member Services desk for $10.
For more information. Check out http://www.pdx.edu/recreation/youth-swim-lessons.