Create Child Work Item with Powershell

Hello,

I have this script, which is working fine, to create a work item at Team Foundation Server.

cls
if ( (Get-PSSnapin -Name Microsoft.TeamFoundation.PowerShell -ErrorAction SilentlyContinue) -eq $null )
{     Add-PSSnapin Microsoft.TeamFoundation.PowerShell } 
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Client")  
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Build.Client")  
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Build.Common") 
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.WorkItemTracking.Client") 
$tfsCollectionUrl = "http://zmuctfsdev:8080/tfs/dev_test_collection"
$teamProjectCollection = [Microsoft.TeamFoundation.Client.TfsTeamProjectCollectionFactory]::GetTeamProjectCollection($tfsCollectionUrl)
Write-Host "Team Project Collection: "$teamProjectCollection
$ws = $teamProjectCollection.GetService([type]"Microsoft.TeamFoundation.WorkItemTracking.Client.WorkItemStore")
write-host "WorkItemStore: "$ws
$proj = $ws.Projects["ms-basis dev"]
$staging = $proj.WorkItemTypes["staging"]
$workitem = $staging.NewWorkItem()
$workItem.Title = "TitleNEW"
$workitem.History = "Staging requested by " + [Environment]::UserDomainName+"\"+[Environment]::UserName
$workItem.Save()

My question:

How can I create a child work item to this new created work item with powershell?

I hope somebody can help me,

Thanks Horst MOSS 2007 Farm; MOSS 2010 Farm; TFS 2010; IIS 7.5