J. A. Darke 
Grin in the Dark [PDF ebook] 

While babysitting his twin cousins one stormy night, Hamid Abdi sees an alert that a prisoner is on the loose nearby. He doesn’t think much of it — until the twins tell him that a man dressed as a clown has been coming into their bedroom. Still, Hamid thinks the twins are imagining it and puts them back to bed. But when Hamid starts seeing and hearing terrifying things as well, will he be able to convince himself it’s all just a nightmare?