"Such people put up a version of Christianity suitable for a child of six and make that the object of their attack."

The type of logical fallacy Lewis is criticizing here is called a straw man argument: "The Straw Man fallacy is committed when a person simply ignores a person's actual position and substitutes a distorted, exaggerated or misrepresented version of that position."

In the UK, such an argument is known as an "Aunt Sally".