How to create a schema for an unordered list of XML nodes, with occurrence constraints

Given an XML layout like this, I'm trying to create a XSD schema to validate it.

<RootNode>
  <ChildA />
  <ChildC />
  <ChildB />
  <ChildB />
  <ChildA />
</RootNode>

The requirements are as follows:

• ChildA, ChildB and ChildC may occur in any order. (<xs:sequence> unsuitable)
• ChildA is mandatory but may occur multiple times.
• ChildB is optional and may occur multiple times.
• ChildC is optional and may occur once only.

The technique I usually use to create an unordered list of nodes is to use a <xs:choice maxOccurs="unbounded"> with each possible node in the list, however, I am unable to create the minOccurs="1" constraint on ChildA and the maxOccurs="1" contraint on ChildC. (The # of occurances of the choice takes precedence over those of the elements here).

<xs:element name="RootNode">
  <xs:complexType>
    <xs:choice minOccurs="1" maxOccurs="unbounded">
      <xs:element name="ChildA" minOccurs="1"/>
      <xs:element name="ChildB" />
      <xs:element name="ChildC" maxOccurs="1"/>
    </xs:choice>
  </xs:complexType>
</xs:element>