The Thought Police are the organization that oversee the young spies and encourage them to report on their parents or their families but it is an outreach of a larger policy on the part of the party itself. They believe that it is important to undermine loyalty to the family in order to encourage that familial loyalty to be turned towards the party instead.
The incentive offered to the young members in terms of becoming a party member and other perks is important to again undermined their loyalty to the family and also provide reinforcement for the power of the party and the thought police.
It is indeed the Thought Police to whom the children betray their parents. This action, though, is supported by the Youth League and the Spies.