I am not sure when the fountains and other water featured are closed, but the swings, climbing "toys" and walking and bike trail open year around. Cincinnati Park Board could probably tell you about your specific question.
Just rode my bike past all the fountains. Today is 21 September, and they were all still operating.
All the fountains were open last weekend if that is helpful
The fountains have typically remained on until the first threats of freezing, from my experience.
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