How to add child nodes to an existing xml file?

I need to add child nodes to an existing xml file.  For example, the existing file is like

<Node1><ChileNode1>a</ChildNode1></Node1>

I have to add more ChildNodes like

<Node1><ChileNode1>a</ChildNode1><ChileNode1>b</ChildNode1><ChileNode1>c</ChildNode1></Node1>

Where can I find sample c# code for adding them?  Thanks.