Indexed (Materialized) View for Parent and Newest Child
There isn't a way to do this currently, as finding the "latest" widget date will require an aggregate (grouping), ranking function, subquery, or apply; none of which the indexed view will allow in its definition.

However - is there a particular reason you want to have an indexed view? Do you have appropriate indexing on the underlying Widget table? I'm assuming that the reason you want to index the view is so you can pull out the values for a particular Widget ID.

I would index the underlying table on (WidgetID, CreatedWhen DESC) and implement your query as a normal view - or perhaps a inline table valued function if you always want to get the date for a single Widget.