You are missing the cc:insertChildren tag.
<cc:implementation>
<p:dialog header="#{cc.attrs.header}" id="#{cc.attrs.id}" >
<cc:insertChildren />
</p:dialog>
</cc:implementation>
Any child components or template text within the composite component tag in the using page will be reparented into the composite component at the point indicated by this tag’s placement within the composite:implementation section.