I think this is what your after...
In the dashboard you can select a page type and click the defaults button. This takes you to an editable page of that page type. Add your menu block here - then click on it and add it to child pages.

However, if you make any changes to the default later, it wont update the child pages. You need to delete it first (which will automatically remove it from all child pages) and then re-add it with your changes.

I hope that makes sense...

Jon :)