Magical Children: The Invisible Boy - Sally Gardner

When his parents are lost in space, Sam is left, heartbroken, in the care of the horrible Hilda Hardbottom. Then he finds a tiny spaceship in the cabbage patch and meets a little alien called Splodge. How Splodge makes him invisible, and how Sam uses his new talent in his darkest hour, makes a touching and extremely funny story with lovely memorable characters.