Based on what posted here, since your code already converting the XML into string, have you thought of using regular expression? Here is an example with the assumption that the string that wrap around the ID_NUMERIC is always identical and only one ID_NUMERIC per XML.
string sampleXmlText= "<field id=\"AscaeeeDddddUCT\" direction=\"in\" datatype=\"Text\">Bla</field><field id=\"ID_NUMERIC\" direction=\"out\" datatype=\"Text\"> 711355</field></fields><children><entity type=\"TEST\"><actions /><fields>";
Console.WriteLine(Regex.Matches(sampleXmlText, "(?<=<field id=\"ID_NUMERIC\" direction=\"out\" datatype=\"Text\"> )(.*?)(?=</field>)")[0]);
Output: 711355