How to ensure at least one child element exists via XSD?

How can I ensure than at least one of location's child elements (locality, wkt) is specified when the location element is included in an XML?

<xs:element name="location" nillable="true" minOccurs="0">
  <xs:complexType>
    <xs:group ref="cs:locationGroup"></xs:group>
  </xs:complexType>
</xs:element>

Definition of locationGroup:

<xs:group name="locationGroup">
  <xs:all>
    <xs:element name="locality" minOccurs="0"/>
    <xs:element name="wkt" minOccurs="0"/>
  </xs:all>
</xs:group>

The version of my XSD is 1.0.

Solution

For such a small number of possible child elements, simply define a xs:choice of the allowed combinations:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="location">
    <xs:complexType>
      <xs:group ref="locationGroup"></xs:group>
    </xs:complexType>
  </xs:element>

  <xs:group name="locationGroup">
    <xs:choice>
      <xs:sequence>
        <xs:element name="locality"/>
        <xs:element name="wkt" minOccurs="0"/>
      </xs:sequence>
      <xs:sequence>
        <xs:element name="wkt"/>
        <xs:element name="locality" minOccurs="0"/>
      </xs:sequence>
    </xs:choice>
  </xs:group>
</xs:schema>

Note that this approach

requires one or both of locality or wkt to be present
allows any order when both are present
works in both XSD 1.0 (and 1.1)

as requested.