A foil is a character who provides a clear comparison. So what you are being asked to do is compare and contrast the Parsons and Winston.
The Parsons are totally devoted to Big Brother and the Party. The reader and Winston know they are highly unlikely to commit a thought crime. Winston alludes to this early in Chapter Two and also points out that any thought crime would really be impossible for them because they have children who are Spies for Big Brother. This foreshadows what is going to happen later in the novel to Tom Parsons when Winston is in a cell with him in the Ministry of Love.
Winston is comparatively open in his "thought crime" as he writes, "Down with Big Brother." The children seem aware of this somehow and run around him yelling, "You're a traitor...You're a thought criminal." By this stage Winston has already told us that he is as good as dead. The Parsons in contrast see themselves as law abiding and completely loyal. We later realise that no one is safe and there is no escape from Big Brother and the Party. Not even in the deepest recesses of one's mind. Winston never had a chance.