Do you expand the tree before you add child items to it? In that case he doesn't get expanded.
Here is an example which works for me:
private Tree createTree() {
// Create a Tree
TreeItem root = new TreeItem("root");
root.addItem("item0");
root.addItem("item1");
root.addItem("item2");
TreeItem childwithsubchilder = new TreeItem("subitmes");
childwithsubchilder.addItem("item0");
childwithsubchilder.addItem("item1");
childwithsubchilder.addItem("item2");
root.addItem(childwithsubchilder);
// expand the element
root.setState(true);
Tree t = new Tree();
t.addItem(root);
return t;
}