Just checked the change history of RootNavigationItem and NavigationItem which it inherits. No changes have been made.
This is how it looks in the source - still public:
Public ReadOnly Property ChildNodes() As List(Of NavigationItem)
Get
Return children
End Get
End Property
Could you specify your issue a little more - it could be something else.
My fault. I changed location for the project some days ago and somehow it was giving me this error like the method was not accessible. Some references or variables must have been static.
I moved the project back and now it works again. Strange behavior by VS2010.