Like mentioned on the wiki when using treegrid methods (getNodeChildren) you need to pass the record using getRowData, but getRowData doesn't return the field (localreader.)id which causes it to fail.

var rd = $(this).jqGrid('getRowData', rowid);
var children = $(this).jqGrid('getNodeChildren', rd);

The way it does work at the moment is using the data property of the grid:

var rInd = $(this).jqGrid('getInd', rowid); // get index for id
var rData = $(this).jqGrid('getGridParam', 'data'); // get all localData
var rd = rData[rInd - 1]; // get record
var children = $(this).jqGrid('getNodeChildren', rd); // getchilren