In XML Schema specification there is requirement that ONE of the following should be true:
------ Spec excerpt (http://www.w3.org/TR/xmlschema-1/#c-fields-xpaths)
2.1
   Selector XPath expressions
   [1] Selector ::= Path ( '|' Path )*
   [2] Path ::= ('.//')? Step ( '/' Step )*
   [3] Step ::= '.' | NameTest
   [4] NameTest ::= QName | '*' | NCName ':' '*'

   Path in Field XPath expressions
   [5] Path ::= ('.//')? ( Step '/' )* ( Step | '@' NameTest )
   [5] token ::= '.' | '/' | '//' | '|' | '@' | NameTest
   [6] whitespace ::= S
   [7] Path ::= ('.//')? ( Step '/' )* ( Step | '@' NameTest )

2.2 It must be an XPath expression involving the child and/or attribute axes whose abbreviated form is as given above.

----- End spec

But SchemaFactory reports error on schema containing XPath expression with "child" axis.

For example, for this schema (Test: xml_schema/msxsdtest/identityConstraint/idL029.html#idL029):
<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" targetNamespace="myNS.tempuri.org" xmlns:myNS="myNS.tempuri.org" xmlns="myNS.tempuri.org">
<xsd:element name="root">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="t" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
<xsd:key id="foo123" name="tableu">
<xsd:selector xpath=".//myNS:t"/>
<xsd:field xpath="child::myNS:row"/>
</xsd:key>
</xsd:element>
<xsd:element name="t" type="ttype"/>
<xsd:complexType name="ttype">
<xsd:sequence>
<xsd:element name="row" type="xsd:string" maxOccurs="unbounded"/>
</xsd:sequence>
<xsd:attribute name="col" type="xsd:string"/>
</xsd:complexType>
</xsd:schema>

Following code:
....
public static void main(String[] args) {
try {
SchemaFactory sf = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
Schema s = sf.newSchema(new File("idL029.xsd"));
} catch (SAXException e) {
e.printStackTrace();
}
}
....

Reports error:

org.xml.sax.SAXParseException: c-general-xpath: The expression 'child::myNS:row' is not valid with respect to the XPath subset supported by XML Schema.
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(ErrorHandlerWrapper.java:236)
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.error(ErrorHandlerWrapper.java:172)
at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:382)
....