using System;

using System.Linq;

using System.Xml.Linq;

​

class Program

{

    public static void Main()

    {

        const string xml = @"

<Root>

    <I aa=""1"">

        <s aa=""3""/>

        <s aa=""1""/>

        <s aa=""2""/>

    </I>

    <I aa=""5"">

        <s aa=""3""/>

        <s aa=""1""/>

        <s aa=""2""/>

    </I>

    <I aa=""3"">

        <s aa=""3""/>

        <s aa=""1""/>

        <s aa=""2""/>

    </I>

    <I aa=""4"">

        <s aa=""3""/>

        <s aa=""1""/>

        <s aa=""2""/>

    </I>

</Root>";

​

        var root = XElement.Parse(xml);

​

        var newRoot = CopyAndSort(root);

​

        Console.WriteLine(newRoot);

    }

​

    private static XElement CopyAndSort(XElement e)

    {

        var elements = e.Elements()

            .OrderBy(x => (string)x.Attribute("aa"))

            .Select(CopyAndSort);

        return new XElement(e.Name, e.Attributes(), elements);

    }

}