Original: Police cars keep the city and its roads safe.
Corrected (simple): Police cars help keep the city and its roads safe.
Why: The original sentence is already correct. It has a clear subject (Police cars), a correct verb (keep), and the objects (the city and its roads) that are being kept safe. Adding the word "help" makes the idea a little easier for a young reader to understand.
Tip for kids: If you mean all places, you can say, "Police cars keep cities and roads safe."