ASAIU, when you for instance traverse a tree of TreeItem:s you have to check if there is any children to traverse into for every TreeItem. Since you need to call TreeItem.getChildren().isEmpty() to know this you will make sure that every single TreeItem in the tree has an associated ObservableList and a Property, as well as a listener. Normally there will be more TreeItem:s that are empty than not.

So, please add

TreeItem#getChildCount()
or
TreeItem#hasChildren()

Note that TreeItem#isLeaf() is not the same thing since that can be set separately.

It would also be very good if you added TreeItem.stream() for visiting a tree structure.