There should be an interface for parents that have a public getChildren() method

There is currently no common parent class or interface with a public "getChildren()" method. That make generic tracking of a parent container awkward in some circumstances. To add children to a node we first need to cast to the appropriate type, Group or Pane or something else. If there were an interface for this method that both Group, Pane, and other potential parents could implement, it could simplify code in many cases.