I've came across the same problem, a while back.
The way I did it was to implement the INavigationAware interface on the Parent ViewModel and in the ViewModels of those scoped Regions/Views and then on the Parent ViewModel OnNavigatedFrom method, call the OnNavigatedFrom of the children ViewModels.
Bear in mind that you have to keep a reference of the Children ViewModels in the Parent ViewModel (which I believe is not a bad practice). This reference can be injected by the Container and deleted (Teared Down) when you navigate away from the Parent View (depending on the value of the KeepAlive property if you've implemented the IRegionMemberLifetime in the parent and children ViewModels).