You should be able to set your Category URI Format to {parent.slug}/{slug}

If they are on the parent it will only show the parent category. If they are on the child it will show both.

So you'd set it to

what-we-do/{parent.slug}/{slug}

You should be able to set your Category URI Format to {parent.slug}/{slug}

If they are on the parent it will only show the parent category. If they are on the child it will show both.

So you'd set it to

what-we-do/{parent.slug}/{slug}

There is also this answer if you need the path beyond 1 level:

{parent.uri}/{slug}