This is mixed content:

<p>Hello <a href="http://www.w3.org">W3C</a></p>

i.e. elements and text at the same level.

There's a simple flag for this:

<xs:element name="p"> 
  <xs:complexType mixed='true'>
    <xs:sequence>
      <xs:element name="my:a" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

But you can't type the text--its just a string (e.g. you can't say its an integer).