The film will be directed by McG, said The Hollywood Reporter.
The New Line movie follows a young boy (Lewis) and his babysitter (Weaving) on a night where things go very wrong, with the boy learning a few lessons about life, love and murder along the way.
McG will produce via his Wonderland Sound & Vision along with the company's president, Mary Viola. Production is slated for later this Fall in Los Angeles.
The script is penned by Brian Duffield.
Amell, 27, plays Ronnie Raymond on The CW's hit series The Flash, and previously appeared in TV series "The Tomorrow People" and "Revenge".
The actor will also appear in "Nine Lives" with Kevin Spacey.