"a boy and a girl"
Translation:egy fiú és egy lány
Maybe not the meaning, more like the usage. I think the same way that it would change in English. It is hard to think of a good example, but maybe one of those general statements would do, as we know that Hungarian likes to make those statements in the singular:
There are boys, and girls, too, in the team.
Fiúk és lányok is vannak a csapatban - plural version
Fiú és lány is van a csapatban - singular version