random optional elements

[lexul]

Is there a way to specify in a xsd schema that i want to allow some optional subtags in any random order(each occuring at most once), and at least one of them(no matter which) to be present?
Example(this should be validated by the xsd schema)

<parent>
<child1/>
<child2/>
</parent>

<parent>
<child2/>
<child1/>
</parent>

<parent>
<child2/>
</parent>