If I understand you correctly, you need to use -SearchBase:
Get-ADUser -SearchBase "OU=Accounts,OU=RootOU,DC=ChildDomain,DC=RootDomain,DC=com" -Filter *
Note that Get-ADUser defaults to using
-SearchScope Subtree
so you don't need to specify it. It's this that gives you all sub-OUs (and sub-sub-OUs, etc.).