This is a question that is asked a lot. This is only possible by custom coding. I do not have any examples. This would require adding custom JavaScript to ACF and using the ACF JS API. It would require altering the AJAX values sent when populating the child taxonomy field. It would also require adding an action to the parent taxonomy field that causes ACF to reevaluate the child. As I said, there are no examples of this because no one has figured out how to do it.
It is possible to use regular select fields, there are examples of doing this for post types, but not taxonomies, but the idea would be the same and you would need to modify them to use a taxonomy. Do a google search for “acf dynamic ajax select example”.
