how to default select a node of a tree panel in extjs 3.4

I have a tree panel which have 10 childrens .I want to select default third child of them. what can i do . I am using this code for select but its not work.

var root = Ext.getStore('MyStore').getRootNode();

var firstChildNode = root.findChild('leaf', true, true);

Ext.getCmp('treePnl').getSelectionModel().select(firstChildNode);