How to unregister a widget from a child theme

Assuming the Parent Theme’s Widget is registered at the widgets_init hook, all you need to do is wrap your unregister_widget() call inside of a function, hook that function into widgets_init, and give your hook a higher priority number than the Parent Theme’s widgets_init-hooked function.

Assuming the Parent Theme hooks into widgets_init without giving a priority number, the function will default to 10. So, just give your hook call a priority of 11 or greater.