The other alternative to the above suggestions, is that you define your own message, and use that to signal to your child window that you've got some data for it. I use the following code to pass an image to a child window. IDC_COPYIMAGE is the message I defined. (#define IDC_COPYIMAGE 1010) There just needs to be an appropriate handler in the windowproc for the child dialog.
if (myWad.getLumpSize(myItem.iItem) == 4096)
{
flatData = (char*)myWad.loadResource(resNum, &resLen);
pal = (char*) myWad.loadResource(195, &resLen);
darkMap = (char*) myWad.loadResource(1, &resLen);
tmpImg = createImageFromFlat(flatData, pal, darkMap);
delete pal;
delete flatData;
delete darkMap;
SendMessage(imgChildHWND, IDC_COPYIMAGE, 0, (LPARAM)tmpImg);
DeleteObject(tmpImg);
}