How can I convert a ContentData object to a ContentReference so that I can get the children of the ContentData object?

So I'm trying to build a recursive tree of ContentData objects starting from the ContentReference.RootPage.  It looks like the IContentLoader method GetChildren would do the trick, but it's only allowing me to send a ContentReference to the GetChildren argument.

If I knew how to convert a ContentData object to a ContentReference object, I could probably make this work.  But I don't know if that's possible.  Here is my latest attempt.

// These 3 lines do what I expect.
IContentLoader contentLoader = ServiceLocator.Current.GetInstance<IContentLoader>();
List<ContentData> contentDataList = contentLoader.GetChildren<ContentData>(ContentReference.RootPage).ToList();
ContentData rootContentData = (ContentData)ServiceLocator.Current.GetInstance<IContentRepository>().Get<IContent>(ContentReference.RootPage);
foreach (ContentData child in contentDataList)
{
    // This next line fails with the error:  Cannot convert from 'EPiServer.Core.ContentData' to 'EPiServer.Core.ContentReference'
    List<ContentData> children = contentLoader.GetChildren<ContentData>(child);
    //ContentReference childRef = (ContentReference)child; // Nope
    //ContentReference childRef = child as ContentReference; // Nope
}

How can I get the children of a ContentData object?  Thank you.