Moving from 7v7 to 9v9 — the pitch suddenly feels huge

Our U11s are starting to get some 9v9 experience and the biggest difference seems to be how stretched everything becomes. In 7v7 we can stay connected quite naturally. In 9v9 the wide players drift miles away and the back line can end up detached from midfield. What would you introduce first so they understand the extra width without filling their heads with a new set of positions and distances?