There is no MDI API in the core JavaFX toolkit.
You can find some relate information in the thread: Like JFrames and JInternalFrames "Like JInternalFrames"
and other similar threads by searching the forums for docking or JInternalFrame
There is a third party MDI style windowing framework in JFXtras http://jfxtras.org/. Try it out. It is likely not fully featured, but perhaps it will meet your needs or provide you a starting point.