"The girls and boys are walking on the streets."

Translation: A lányok és fiúk az utcákon sétálnak.

[j0hanney]

Is it wrong to add "a" in front of "fiúk" as well?

[jsiehler]

No, it's not wrong. It's about the same as English: You can say "The girls and boys" or "The girls and the boys." I suppose since they omit the second "the" in the English version, it's reasonable to omit the corresponding word when translating to Hungarian for a good match.