Please make sure that you are using the latest released assemblies from our last internal build.
I tried the example with the following code and it works as expected:
private void Application_Startup(object sender, StartupEventArgs e)
this.RootVisual = new Page1();
ChildWindow window = new ChildWindow();
RadHtmlPlaceholder placeHolder = new RadHtmlPlaceholder();
placeHolder.SourceUrl = new Uri("http://www.telerik.com",UriKind.RelativeOrAbsolute);
window.Content = placeHolder;
Please let us know if you have any other questions or need some more help or sample code.
the Telerik team