using System;

using System.Linq;

using System.Xml.Linq;

​

public class Program

{

    public static void Main(string[] args)

    {

        const string xml =

            @"<SomeParentElement>

  <SomeChildElement/>

​

  <SomeChildElement2/>

  <SomeChildElement3/>

</SomeParentElement>";

​

        var doc = XDocument.Parse(xml, LoadOptions.PreserveWhitespace);

​

        var child = doc.Elements("SomeParentElement").Elements("SomeChildElement").First();

        child.AddAfterSelf(

            new XText("\r\n  "),

            new XElement("TestElement"));

​

        Console.WriteLine(doc);

    }

}