Sorting parent and child as one

I have a document with parent (CAT) and child (LI) nodes, each with a TXT
attribute.  When these attributes get compiled for output, they need to be
sorted.  When they get sorted, the parent/child association needs to be
ignored.  Currently, they are either sorting by one or the other first...to
make the full list work, it needs to treat them as one and the same.  How can
I treat the parent/child TXT attributes of the following XML as the same, or
somehow ignore the relationship, for sorting?

<DMML>
 <LST>
  <GRP TXT="Automobile Codes" VTY="\X">
   <CAT COD="AAAA" TXT="American Motors (Rambler prior to 1966)">
    <LI COD="MMM" TXT="American Motors Medallion (thru 1987)"/>
   </CAT>
   <CAT COD="XXXX" TXT="American Motors JEEP (use truck code)">
    <LI COD="XXXX" TXT="Buick Opel (Imports)"/>
   </CAT>
   <CAT COD="BBBB" TXT="Amphicar"/>
   <CAT COD="CCCC" TXT="Certificate of Origin or Title (for use when make is
not listed or when certificate is blank)"/>
  </GRP>
 </LST>
</DMML>