Welcome to Weirdsville: Happyland: Book 1 by I.M. Strange
On Toby's eleventh birthday, he and his friends get a mysterious invitation to Happyland, the abandoned funfair in town. It's too good an opportunity to miss, even though the place gives them the creeps.
What they find there is more terrifying than any of them could have imagined. Getting in was the easy part. Surviving long enough to escape will be much harder.
Once upon a time Happyland was filled with laughter. Now it's filled with fear.