The Daydreamer (English, Paperback, Ian McEwan)

Peter Fortune is a daydreamer. He’s a quiet ten year old who can’t help himself from dropping out of reality and into the amazing world of his vivid imagination. His daydreams are fantastic and fascinating – only in the bizarre and disturbing world of dreams can he swap bodies with the family cat and his baby cousin, Kenneth, or wipe out his entire family with vanishing cream.
Source- Goodreads.com