parent/shell.nix:

{pkgs ? (import <nixpkgs> {})}:

with pkgs;

stdenv.mkDerivation {
  name = "foo";
  buildInputs = [ baz bar ];
  SOMENV = "..";
}

child/shell.nix:
with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "some";
  buildInputs = [ (import ../shell.nix) some more stuff ];
}