I suggest adding support for the automatic sorting of a Parent's children. 
Motivation: In our app we show a map with pins marking places of interest. A pin which is lower on screenshould always be in front of a pin which is higher, creating the illusion that pins to the south are in front of pins to the north.
Since the children of a JavaFX Parent are not sorted, we have to trigger sorting by listening to changes to the list. This works, but I think our code would be a lot simpler, if we could just invoke

Comparator comparator = new Comparator() {...};
someParent.setChildrenSort(comparator);
someParent.setChildrenSortEnabled(true);

Moreover, such a default implementation could be written in a performance optimising way which would group multiple list changes into a single sort, if you added a method
Parent.setChildrenSortInterval(Duration);

For us to be able to still write list change listeners, we would also require the ability to distinguish between ordinary list changes and sorting permutations, so a method like
boolean Parent.isSorting()
might be relevant.