To answer this question, think about what children are like. Children are interested only in their own immediate gratification (more or less) and do not really think much about anything beyond that. The World State wants its citizens to be like this (especially in the lower castes) so that they do not become unhappy and rebellious.
The World State wants stability more than anything else. To achieve stability, it wants to make sure that its citizens are content and do not want to make any changes. To do that, it infantilizes them because that encourages them to think only of immediate pleasures (easily provided by the state) and not of anything else.