Looks like yesterday the script was not "really" saving. So it wasn't running.
Added a second script to switch to and it asked me to save.
When I did that, I got an error:
The gui for child layer did not change. But, I can move parent now and child follows.
However, if it I close the compo and re-open, I get the error again and it doesn't move with parent anymore.
This may be related. If I manually assign the parent layer to the child layer Link To - it looks like this:
Is there a way to assign Parent -> single in the script? Is that why I get the error? It's looking for a specific way to refer to the Parent layer?