I can confirm swim diapers are required but you can buy one at the desk for a modest amount if you forget to bring one.
The toddler polices are described in this brochure: https://www.seattle.gov/Documents/Departments/ParksAndRecreation/Swimming/AquaticsGuide_2017.pdf
Any time there is family swim. I believe they need swimming daipers if they aren't potty trained yet
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