Please see the last comment in GetListChildren() function. "Why can a List(Of Page) not be converted to an IEnumerable(Of IListHasChildren) ?"

Interface IListHasChildren
    Function GetListChildren() as IEnumerable(Of IListHasChildren)
End Interface

Public Class Page : Implements IListHasChildren

    Public Function GetChildren() As List(Of Page)
        //'return List(Of Page) with children
    End Function

    Public Function GetListChildren() : Implements IListHasChildren.GetListChildren
        Return Me.GetChildren() //'This cannot be converted to IEnumerable(Of IListHasChildren) ??
    End Function
End Class