Questions & Answers about हम समय पर शहर पहुँचेंगे, और वहाँ माँ से मिलेंगे।
समय पर means on time. Literally, पर is a postposition meaning on/upon, so Hindi expresses this idea as something like on time rather than using a single adverb.
This is also a good example of how Hindi uses postpositions after nouns, where English uses prepositions before nouns.
With the verb पहुँचना meaning to reach/arrive, Hindi often uses the destination directly: घर पहुँचना, स्कूल पहुँचना, शहर पहुँचना.
So शहर पहुँचेंगे is a normal way to say will reach the city.
Using में would change the feel a bit, making it more like being located in the city rather than simply reaching it. को is not used here.