Currently TWebTreeView maintains an Items collection, but this is just a collection of root nodes of the treeview, not any of the children. So there's no way to traverse all of the nodes in the treeview using Items. This is not good as it doesn't mirror what the VCL does.
Similarly, the GetNext function of TTreeNode isn't implemented, so traversing nodes that way also doesn't work, another difference between the TWebTreeView and the VCL.
As a workaround, I maintain a separate array of TTreeNode values added to TWebTreeView so that I can then find non-root nodes later.