How to allow/impose elements to occurs one time in any order. With Schema langauge (XSD)

The XML has a parent element "Parent", which have to contain 10 chils: child1 to child10 in any order, but exactly one time.

As an alternative, the parent element can optionally contain child11 to child20 instead of child1 to child10.

<parent><child1/><child4/> ... <child2/></parent> OK
<parent><child5/>...<child20/></parent> OK
<parent><child1/><child2/></parent> BAD, missing childs
<parent><child1/>...<child11/></parent> BAD: child11 shall never be with child1
<parent><child1/><child1/></parent> BAD: childs shall not be repeated

XSD tries:

<xs:complexType name="parent">
    <xs:choice>
        <xs:sequence>
            <xs:element name="child1" type="xs:integer" minOccurs="1" maxOccurs="1"/>
            <xs:element name="child2" type="xs:integer" minOccurs="1" maxOccurs="1"/>
            ...
        </xs:sequence>
        <xs:sequence>
            <xs:element name="child11" type="xs:integer" minOccurs="1" maxOccurs="1"/>
            <xs:element name="child12" type="xs:integer" minOccurs="1" maxOccurs="1"/>
            ...
        </xs:sequence>
    </xs:choice>
</xs:complexType>