Indeed. Nice suggestion. And that would be `Select()` or `Where()`. It took me some time figuring this out, that was fun digging though. I never worked with SelectMany before.

    childrenClassNames = element.Children.Select<ElementNode, string>(child => child.GetAttribute("class") == null ? "" : child.GetAttribute("class").Value).ToList<string>();

and 

    childrenClassNames = element.Children.Where(child => child.GetAttribute("class") != null).Select(child => child.GetAttribute("class").Value).ToList();

I chose the latter one for verbosity. :)