My understanding is that dejar means "to let" and permitir means "to permit."
Either of them can carry the meaning of "to allow," which is to say that they can be used interchangeably as follows:
No deje que los ninos jueguen.
No permita que los ninos jueguen.
The preposition que and the subjunctive jueguen are used to say, "Don't let the children play."
Are there constructions in which dejar and permitir are used DIFFERENTLY, and cannot be interchanged?