There is nothing obviously wrong with the FXML you present.
Oracle have a tutorial for creating a custom control and FXML. The tutorial links a project with source code. Read the tutorial, download the sample source and study it, then apply what you learned to your situation. The sample is really a small amount of code, but I don't want post it here because its Oracle's.
Note, you can use your custom pane from FXML in SceneBuilder, see the SceneBuilder documentation.