Simple way to get element by id within a div tag?

Question

Please forgive me if I repeat the question.

I have HTML　that all elements inside a div tag has different id, suppose I have already get the reference to the div, is

Answer 1

var x = document.getElementById("parent").querySelector("#child");
// don't forget a #

or

var x = document.querySelector("#parent").querySelector("#child");

or

var x = document.querySelector("#parent #child");

or

var x = document.querySelector("#parent");
var y = x.querySelector("#child");

eg.

var x = document.querySelector("#div1").querySelector("#edit2");