The _BUUnsubscribes data view is and thus the query should, be in Parent BU. Which it already is in your case.
a) Does your sending (child) BU have the setting under admin / Business Units set to "unsubscribe from this BU only"?
b) does it have processes in place that perform logUnsubEvents on that Child BU level? You should at least get list unsubscribes, unless you send transactionally classified communication only.
Note also that in contrast to _Unsubscribes, the _BUUnsubscribes Data View does not hold Events, but statuses, so you need to have someone currently on status "unsubscribed" in order to see them appear there. - see here Query Unsubscribes from a specific MID during a specific Period