Ok, here’s what I’m thinking.
I’ve looked at the documentation, and it specifies that triangles can appear within both <convex_mesh> and <mesh> zero to unbounded (0 - *) number of times.
It states in the schema file (found here: http://www.khronos.org/files/collada_schema_1_5) that…
<xs:choice minOccurs="0" maxOccurs="unbounded">
… appears as a parent to…
<xs:element name="triangles" type="triangles_type">
… for both <mesh> and <complex_mesh>.
Which leaves me thinking that you’re questioning that the <xs:element> type in the schema. In this case, I’m out, as I’m not an XML pro. Hopefully one of the pro’s will sign in and help you with your problem.
All the best,