Getting child by instance name behaviour has changed?

I'm not sure if I'm doing something wrong or what, I only use actionscript for very basic stuff. However, how I've always referenced child movieclips is simply like this:

parentMovie.childMovie.secondChild

Since fresh installing and updating to the newest Animate, when I create a new document and try to use this method, it no longer works, I get the error:

A term is undefined and has no properties.

If I trace the parentMovie movieclip, I'll now get output, something like: [object parentMovie_2] BUT if I trace the same thing in my old file I get: [object MovieClip]. Tracing any children of parentMovie in the new file will return "undefined" or "null".

So, if I open a file I created before the update, this method still works as it used to, any new files I create this method does not work. I've looked through all of my properties for the two files and can't see any differences. Any ideas? I've spent hours trying to figure this out without any progress.

Thanks!