What is the ruling on a third prostration if a child pulls the worshiper to prostrate after completing the two prostrations, should the prostration be completed to silence the child or should the worshiper leave him?

A third prostration is not legislated, and intentionally performing it invalidates the prayer. The worshipper should avoid whatever distracts him from his prayer, such as a child, because prayer is an obligation on the adult, and the child is not obligated by it, even if he is commanded to perform it at seven years of age.