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.
For example:
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.