Surprising, because this is really not a a case which is much more
common than others.

But before going into details, you are actually not appending anything
to a JTree. A JTree is just a visual representation of you model. You
change the model, and let the JTree pick up the changes and display the
new data. The distinction is important, as I will point out in a second.

In three steps:

    Remove child2 from its parent
    Add child2 as child to newNode
    Insert newNode at the desired position

Other sequences are also possible (e.g. first adding newNode, then
removing child2, than adding child2 to newNode).

The important thing is that the JTree is properly informed about the
changes in the model once the changes are done. You can either

* perform all operations via the TreeModel. Which, if correctly
implemented, will inform the JTree, or you can

* rearrange the nodes and notify the JTree afterwards.

Rearranging the nodes and notifying the JTree afterwards has the
advantage that you can limit the number of notifications which are fired
to the JTree. If you go via the model, you probably trigger two events
(for the remove and the insert).

So a code sequence which just fires one event would look something like

    DefaultMutableTreeNode parent, child1, child2, newNode;
    DefaultTreeModel tm;

    int ndx = parent.getIndex(child2); // remember where to insert
    parent.remove(pos);
    newNode.insert(child2, 0);
    parent.insert(newNode, pos); // reinsert at old pos. of child2

    // let the model inform the JTree about changes below parent
    tm.nodeStructureChanged(parent);

/Thomas