The feeder idea is nice,but the thought of young kids and adults with my hens would not make me a happy personYou can not always stop a child before they do something unpleasant.. I would not want them coming in the yard at all.A kid could get sick from anything,and clueless parent blames it on the chickens.Glad you caught them.Signs and a low volt electric fence around the coop/run.
I wouldn't want anyone feeding my birds without permission... also, why is grass clippings bad? Sometimes when I visit my friend, I pull grass for her horses... how is that different?