いや、こうか。
Get-ChildItem -LiteralPath <親フォルダ> -Directory | %{
  $file = @($_ | Get-ChildItem -Force -Recurse -File)
  [PSCustomObject] @{
    Path     = $_.FullName
    LastWriteTime = ($file | measure LastWriteTime -Maximum).Maximum
    Length    = ($file | measure Length -Sum).Sum
  }
} | ConvertTo-Csv -NoTypeInformation