Of the comprehensive Big Brother system that monitors the thoughts and actions of Oceania's citizens, the child-spies are an important component. The children, Orwell writes in 1984, "were systematically turned against their parents and taught to spy on them and report their deviations. The family has become in effect an extension of the Thought Police. It was a device by means of which everyone could be surrounded night and day by informers who knew him intimately" (133). Thus they did much more than simply "keep an eye" on them. The child-parent relationship is one of intense emotional connection. The very survival of the human species, at least before the time of Oceania, depended on it. The children know their parents better than others and thus can report on even subtle differences in their behavior. Some adults were especially concerned that they might utter something in their sleep, a betrayal of their sub-conscious, and have it be overheard by the telescreen or a child.
What is the motivation for the child to give up a mother or father? They were rewarded with public acknowledgement. The government would honor them as a hero whose selfless act helped protect their nation. Thus the promise of this recognition keeps the kids alerted to the most senitive inclinations of their parents.
No, the Spies are not just the kids who inform on their parents. All kids are supposed to do that. Rather, the Spies are a youth organization run by the Party. It appears that a person becomes a Spy at a young age and then later graduates into the Youth League. So the Spies are supposed to be something like the Hitler Youth -- a group that is meant mainly to indoctrinate the young in the ways of the Party.
You can see hints about this in Chapter 2. We are told that the Spies have been hanging banners for Hate Week. We also see the Parsons' son, who is nine years old, wearing his Spies uniform. This shows us that the Spies are a formal group that do things other than spying on their parents.
Here's a line from Chapter 6 that tells shows the function of the Spies:
By careful early conditioning, by games and cold water, by the rubbish that was dinned into them at school and in the Spies and the Youth League, by lectures, parades, songs, slogans, and martial music, the natural feeling had been driven out of them.