Christophe, if you have to stick to XSD 1.0, a weak and very limited workaround for your problem could be the use of a substitution group:

(a) Introduce a global element declaration with an arbitrary name (e.g. "FoeElem") and type "Foe"

(b) For each element which you would like to allow at the critical place in type "parent": create a global element declaration with type "Foe" (or a type derived from it) and make it a member of substitutionGroup "FoeElem" (substitutionGroup="FoeElem")

(c) In your type "parent" type replace <xs:any type="Foe"/> by <xs:element ref="FoeElem"/>

It's a far cry from a wildcard constrained to have type Foe, as you have to forsee all allowed element names at XSD design time; but you can at least create a set of elements allowed to appear at this place. Note the difference to a choice: a choice is closed, but using a substitution group, additional elements can be allowed later without changing type "parent": by just adding more element declarations declaring this substitution group. Later addition might also be achieved by including or importing further XSDs providing the additional element declarations.

Cheers,

Hans