Invalid XML character error when indexing XML node

All

I have an XML document of the following form:

<RootNode>

     <ChildNode>

          <ChildA/>

          <ChildB/>

          <ChildC/>

     </ChildNode>

     <ChildNode>

          <ChildA/>

          <ChildB/>

          <ChildC/>

     </ChildNode>

</RootNode>

What I need is a means to insert a new node underneath one of the <ChildNode> elements.  However, this requires indexing to specify which <ChildNode> I'd like to add to.  So, using a SetValue, I have the following left-hand side of an expression:

/process_data/xmlVar/RootNode/ChildNode[2]/NewNode

and on the right-hand side of the expression:

'test'

However, the above throws an XML parsing error stating that an "invalid XML character was specified".  If I simply remove the indexing from the left-hand side to yield:

/process_data/xmlVar/RootNode/ChildNode/NewNode

the SetValue properly adds the node <NewNode> but it does so under the first<ChildNode> - which I understand.

So my question is simply:  "How does one index their XML expression properly when trying to dynamically build XML?"  I have tried putting both single quotes and double quotes around the index but to no avail.

Thanks in advance.