In 1984 the party is constantly brainwashing the people of Oceania. They are trying to make them believe that the party is wonderful, that Big Brother is amazing and that everyone should love, and be loyal, only to the party. The way the party brainwashes people is quite effective. They start young, the spies, the youth leagues, these are all ways for them to get to children, to get them on their side, to turn them into obedient little party members. The Spies reminds me a great deal of the Hitler Youth. They have the same philosophy, grab them when they're young and impressionable, and then stuff them so full of propaganda that they believe what ever you tell them.
You see the effects of this childhood brainwashing in Julia. She has been subjected to the party's propaganda all of her life and in some ways she is far more immune to it than Winston, but in other ways she believes just what they want her to. Julia says that if you follow the important rules, go to the rallies and marches and such, you can break the little rules without worrying too much. Winston on the other hand, before he met Julia, would just as soon skip his evening at the community centre as go and be an obedient party member. However Julia doesn't seem to have a problem with the way the party changes the past. How really there is no concrete past only a constantly changing present. She seems to think it is inevitable and nothing particularly important, where as Winston is terrified by it.
There are other reasons for having organizations like the spies. When the party gathers children into groups like the spies they are creating a network of miniature thought police. Once they have been taught to believe the party knows all they are no longer loyal to anyone but them. Children will willingly denounce their family members to the thought police. That is the ultimate horror to have your own children betray you.