Need to Modify Powershell Script to Reset Passwords for Active Directory Users in Child Domain

I have pasted below a Powershell script which resets users passwords and forces to change at next logon.  
The script uses a csv file for the list of users.  
The script worked in the test environment but the production environment has a child domain where the scripts intended users reside.
What changes must be made in order for this script to be able to reset passwords for users in a child domain and force their password to change at next logon?

*** I cannot use Quest cmdlets***

Import-csv users.csv | foreach {
$Searcher = New-Object DirectoryServices.DirectorySearcher 

  $Searcher.Filter = "(sAMAccountName=$($_.Username))" 
 
  $Result = $Searcher.FindOne() 
  $User = $Result.GetDirectoryEntry() 
 
  $User.SetPassword("Password!") 
  $User.Put("pwdLastSet", 0) 
  $User.SetInfo() 
}