You need to reference at least these two namespaces:
using System.Linq;
using System.Xml.Linq;
And try this code:
var xml = @"
<parent>
<child>
<tag>AAA</tag>
<att>BBB</att>
</child>
<child>
<tag>CCC</tag>
<att>DDD</att>
</child>
</parent>";
var xDoc = XDocument.Parse(xml);
var query = from i in xDoc.Root.Descendants("child")
select new
{
Tag = i.Descendants("tag").First().Value,
Att = i.Descendants("att").First().Value
};
var items = query.ToArray();
Remember, that you can use any LINQ extension methods, like Select, Single, Count, Sum, etc.