move the instance declaration before the procedural code.
module tb;
initial begin
childclass c1;
parentclass p1 = new;
p1.set(55,'h900);
p1.get;
p1.set_outside(66, 'h1000);
p1.get_outside();
// IS SOMETHING WRONG WITH THIS LINE???
//c1.set_rd_wr("READ");
//c1.get_rd_wr;
end
endmodule