The Lion And The Unicorn by Shirley Hughes

As Lennys father goes off to fight in the World War II he gives his son a brass badge with a lion and a unicorn on it. Lenny keeps it with him when he has to be evacuated to a big house in the country. He misses his parents and the other children there and at school are spiteful in teasing him. But he finds a secret walled garden to escape. Paperback softback