The Person Controller by David Baddiel

Fred and Ellie are twins. They like the same things, especially video games, which they are really good at! They aren't very good at anything else though!

Then they meet the mystery man, who sends them a video-game controller unlike any other they had ever seen, and it doesn't control their usual games,

When the twins find out what it does control, it seems like the answer to all their problems. The key to their wildest dreams!