According to the Diccionario Panhispánico de Dudas, in Spanish we use the masculine form to talk about collectivities, no matter the gender of its members.
Los alumnos may refer to either a group of exclusively male students or to a group of male and female students.
Los hombres prehistóricos may refer to both prehistoric men and women.
In your example,
Ellos is the recommended article to talk about a group formed by a girl and a boy.
Nosotros should be used as well when talking in the first person plural about a group with both masculine and feminine members.
That was, let's say, the official and recommended use of gender words regarding collectivities. However, I must say that nowadays it's becoming more and more common to use gender-inclusive forms of speech that require the use of both gendered articles when talking about groups:
las y los alumnos;
las y los españoles...
The Real Academia Española —the preeminent council that tries to regulate the use of Spanish— does not approve that approach.