Todos los/as niños/as deberán entregar esta documentación.
The idea is that in this way, “las niñas” (the girls) aren’t left out of the picture. However, according to the rules of the Real Academia Española these constructions are incorrect.
These clarifications are artificial and unnecessary from a linguistic point of view. When dealing with nouns designating living beings there is the possibility of the generic use of the masculine form to make reference to the entire group or species, without distinguishing between sexes.
The explicit use of the feminine form is justified only when the opposition of sexes is relevant in the context:
El desarrollo evolutivo es similar en los niños y las niñas de esa edad.
Indiscriminate splitting between masculine and feminine forms goes against the economy principle in language, as it creates syntactical and concordance difficulties as well as bungling the reading and writing of texts.
The generic use of the masculine form is based on its condition of unmarked term in the masculine/feminine opposition.
For that reason, it is incorrect to use the feminine form to refer to both sexes, irrespective of the amount of people of either sex forming a group. Hence, “los alumnos” would be the only correct way to mention a mixed group of students, even though the “alumnas” may be superior in number to the “alumnos.”
I wonder what the linguists out there think about this issue…?