Relative pronoun

I am interested in the reasoning for using "los cuales" in "no se sabe si permitirán a los niños a llevar sus móviles apagados dentro de sus mochilas, los cuales podrán encender al final del día".  My understanding is that "que" is used by default when referring to a specific noun, in this case "sus móviles", while "el/la/los/las cuales" is an optional, more formal alternative.  But in my answer the simple "que" was deemed incorrect.  Is this because the "que" would refer to "sus mochilas", being the noun immediately preceding, so the "los cuales" is required to disambiguate?  Or have I misunderstood something more basic?