can't serialize ViewPort3d child elements...

hi,
i have a 3d-land thing going, and i want to serialize all the objects in the
scene with XamlWriter. the objects are added as children of a ViewPort3D.
i tried serializing ViewPort3D.Children which is a Visual3DCollection but
this can't be deserialized because it has no default constructor (the
XamlReader barfs with a XamlParseException).

so instead i tried serializing eaech of the child elements individually,
like so:

using (StreamWriter sw = File.CreateText(d.SafeFileName))
foreach(Visual3D v in this.mainViewport.Children)
XamlWriter.Save(v, sw);

and this saves it alright, but it won't deserialize because "there are
multiple root elements". i tried manually adding an <xml> root element but
it got very messy to deserialise.

what is the correct approach? i don't really want to serialize out the
entire window contents, because the code behind will change with new menus /
options etc., and i will want to be able to use old xaml files from a
previous version.

thanks in advance
tim