we have sinks in our gardens that are made for camping and fishing from Amazon:
it costs $149, plus we purchased hoses suitable for potable water and attach them to an outdoor faucet directly. They can be turned off of course, and only turned on when in use. This should meet the standards of your health department with some procedures in place if you want to go that far for approval. Kids can also wash their hands, we use a bar of soap in a stocking tied to the table, it lasts quite a while. This also enables kids to wash off their radishes before consuming in the garden if that's what you would require. The waste water runs into a bucket which can then be used to water the garden.
judy
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