Silverlight 4 DataGrid Row Detail Issue

Question

Hi,

I am trying to create a Row Detail in DataGrid in Silverlight 4.I have a DataSource as

Public User parentUser{get;set;}

Public List childUsers{get;set;}

In the First Grid I have to display all the parentUser details and In the detaisl Grid I ahev to display all the childusers associated with the parentUser.

   <sdk:DataGridTextColumn Header="Description" Binding="{Binding Description}" SortMemberPath="ActvityValue"/> 

<sdk:DataGridTextColumn Header="First Name" Binding="{Binding FirstName}" SortMemberPath="FirstName"/>

<sdk:DataGridTextColumn Header="LastName" Binding="{Binding LastName}" SortMemberPath="LastName"/>

<sdk:DataGridTextColumn Header="LastName" Binding="{Binding ChildUsers.LastName}" SortMemberPath="LastName"/>

....

By the above way I am only able to display the parent rows and non of the details show up..

i AM NOT ABLE TO UNDERSTAND WHERE i AM DOING WRONG.

i REQUEST SOME SOLUTIONS AND SUGGESTIONS PLEASE.

Thanks in Advance.