Parallelism -- and unneeded repetition. If "killed by guns" toward the end of the sentence is removed, the sentence reads mor smoothly.
*more smoothly*
The sentence you provided is an example of parallelism. Parallelism is a literary device that involves using similar grammatical structures or patterns to create balance and rhythm in a sentence. In the given sentence, the structure of the comparison is parallel, as it compares two different statistics using the same grammatical structure: "more preschool-age children are killed by guns each year than police officers are killed by guns."
