How can I specify an element to have an attribute that states how many children it contains in an XML Schema?

Question

Is it even possible?

• I know it’s possible to do a restriction based on regex, but that’s not it
• I know it’s possible to declare an attribute as a foreign key calculated by an XPath, but it seems it has to be unique

Exemple:

<root children="2">
    <child />
    <child />
</root>