The getChildrenResponse type does not allow mixed content.
There is no restriction on type derivation.
XML Instance
<...>
List of requester definitions that are immediate children of the supplied
requster group. Each definition has a definition type that indicates whether an entry
in the list describes an individual requester or a requester group.
<xhtml:p>List of requester definitions that are immediate children of the supplied
requster group. Each definition has a definition type that indicates whether an entry
in the list describes an individual requester or a requester group.</xhtml:p>