Welcome @SousageMouse !
You don’t need to use signals, or anything like that.
You could extend the Parent.save() method and have it call an update method (when necessary) on the child models. (Then, you only need to ensure that all updates to Parent use that save method.)
If you need to ensure that this will always be done under all circumstances, then you would need to write a trigger on the database to force the update to occur at the database layer.