Hi there Adam.
Right now, the best way to do this would be to create your own widget in the child theme, but if you’re going that route, you may as well hard code a menu into your child theme.
Another option would be to use a pre-made menu widget. I haven’t tested any of these, but if you’d like to give them a go, let us know what you find.
http://wordpress.org/plugins/dropdown-menu-widget/
http://wordpress.org/plugins/advanced-menu-widget/
The first one seems like it’s closest to what you’re after.