The phrase "boy running" can be translated directly as "男孩跑步" in Chinese. In English, it is simply "boy running." For example, you might say, "The boy is running in the park" to describe the action.

If you're looking for a more formal or alternative expression, you could use "young male sprinting" or "lad jogging," depending on the context. These variations convey slight nuances in speed or formality.