Auzou Livre pour moi, arthur, maître du royaume perdu tome 1

When Arthur and Rose were little, they were the heroes of the Roaring Kingdom, a world imagined in their grandfather's attic. This Kingdom was full of things they loved: dragons, mermaids, ninja wizards, and things that scared them: especially a particularly terrifying scarecrow with crow wings.
Today, the twins are eleven years old, and the Roaring Kingdom is just a distant memory. Arthur and Rose no longer play together. Rose is too cool for that, and Arthur is too anxious about starting secondary school! But as they help their grandfather clear out the attic, Arthur, horrified, sees it vanish. Is he playing a trick on them?
Unless the Roaring Kingdom is... real?