Well, "Tools" -> "Select Elements" offers the option to select all child nodes of selected nodes. Additionally, said tool also offers the option to select labels of selected nodes. This means it should be possible to select the labels of child nodes by running the tool twice with appropriate settings.
However, "Select Elements" will never select elements in folder nodes.
Moreover, I do not think there features in yEd that take selected elements in folder nodes into account.
Can you explain what you are actually trying to do (i.e. why you would like to select elements in folders)? Maybe there is a better way than trying to work with the elements in folders.