That’s currently not an option, and though it would probably be possible to add this as a feature it would probably require another set of options (another dropdown box). Since it’s possible to achieve what you want with the current setup (see below) it probably won’t be a future feature.
The Static Page options are select the Pages you want a widget to be shown/hidden on, so to achieve what you want would mean selecting the Parent AND all the Child Pages. Biggest hassle of this is if you add a new Child Page you have to go add it to the Widget Options.
David