need to determine if popup is loaded

Here's the situation,
I have a window(lets call this the parent window) which displays a set of images. The user can popup a window(child window) to get a better display of the images. Now, the user has an option to close the parent window and open another one(this will become the new parent window) to display another set of images without closing the child window. With this, the child window must also be refreshed in order to load the same set of images. The thing is, these 2 windows must be synchronized, so i need to change the reference of the parent in the child window. Im doing something like this:

// in the parent window
var win = window.open(url,name);
win.setParent(window);

//in the child window
var parentWindow = null;
function setParent(win){
     parentWindow = win;
}

The code works fine.. BUT the problem is when the child window takes long time to refresh, the parentWindow becomes null. I think that's because win.setParent(window) has been executed while child window is still loading. Obviously, i must call setParent() right after the child window is finish loading. So I've tried something like this in the parent window:

var win = window.open(url,name);
win.document.body.onload = childOnload;
function childOnload(){
   win.setParent(window);
}

But this code doesn't seem to work. Is there any other way to determine if popup window has been loaded?Or am I missing something on that code? Btw, I've also tried win.onload = childOnload; and didn't work.