JavaScript How to address children and grandchildren without id or class?

[Till]

I have a parent element with the id `39`. I aim to change the z-index of a specific grandchild without id or to increase the z-indexes of all grandchildren. How can I achieve this?

My JavaScript event:

<div onmouseover="hihi()">

My working function to change the z-index of the parent element with the id `39`:

function hihi() {
      document.getElementById("39").style.zIndex = "9999";
    }

The grandchild:

<div id="39">
  <div>
    <div>
    </div>
  </div>
</div>

My non-working function to change the z-index of the grandchild of the element with the id `39`:

function hihi() {
      document.querySelector("#39 div div").style.zIndex = "9999";
    }

My non-working function to change the z-index of all grandchildren of the element with the id `39`:

function hihi() {
      document.querySelector("#39 *").style.zIndex = "+9999";
    }