BEIJING, July 21 (Reuters) - The Cyberspace Administration of China (CAC) said on Wednesday it summoned representatives of Kuaishou (1024.HK), Tencent"s (0700.HK) messaging tool QQ, Alibaba"s (9988.HK) Taobao and Weibo (WB.O) for spreading child-related obscene content. CAC said in a statement it ordered the platforms to "rectify" and "clean up" all illegal content, and that CAC would fine the platforms.