Changing subpage permalinks

Does anybody know of a way to change (or a plugin that does it) the permalink structure for subpages. Currently wordpress does the following:
https://www.mysite.com/parent-page/ < Navigational parent of sub-page
https://www.mysite.com/parent-page/sub-page < Navigational sub-page of parent-page

I would like to have the following:
https://www.mysite.com/parent-page/ < Navigational parent of sub-page
https://www.mysite.com/sub-page < sub-page of parent-page

I need the sub-page to be a sub-page for the navigational/drop-down menus, but in the URL I don’t want it (for SEO reasons) to be after /parent-page.

I know there won’t be any URL clashes etc for my site if I do this.

Anybody know a way of doing this?

Thanks