The Runaway Troll by Matt Haig
There's one thing every troll child dreads more than anything - and that's being sent to the Betterer. The Betterer is the most evil troll in Shadow Forest who loves to punish his fellow trolls for their grubby and stupid habits in lots of horrible ways. When one troll boy escapes the forest to be with his favourite human, Samuel Blink, the Betterer is not very happy. Until he realizes that the most useful thing to have in his Bettering Tower would be a real life human child. So he places his Bettering Branch and Gripgrass Juice in his pockets and sets out on a quest into the human world beyond the forest to kidnap Samuel and the troll friend he's been hiding in his wardrobe.