Walden Pond is probably one of the closest places to swim with a beach. As far as kids activities, there's plenty for kids to do at Walden. Gore Place is museum not too far with animals. Pizzi Farm is a place nearby with ice cream. Drumlin Farm isn't too far. If your question is about the Cambridge Reservoir right next to the hotel, the answer is that there are no beaches. It also isn't a very walkable area.
Ocean Beach no. Possibly a lake I'm not certain about that. As for kids activities in Boston there's ton like duck boats, public garden science museum, aquarium, etc. In Concord a short drive away there's drumlin farm and decordova.
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