Whether some of the children in each group chastised children who punched the doll doesn’t weaken the politician’s argument, which relies on the observation that most of the children who watched the film did punch the doll.

Even if a child who didn't watch the film punched the doll the hardest, the fact remains that most children who watched the film did punch the doll, while most who didn't watch it did not. So (B) doesn’t weaken the politician's argument.

The politician aims to address children hurting other children. For his conclusion to hold, he must assume that children who punch the doll will also hurt other children. But if children who watch the film are no more likely to punch other children, his argument falls apart.

Like (B), it doesn’t matter that some of the children who didn’t watch the film later punched the doll, because the politician’s argument relies on the fact that most children who watched the film did punch the doll, while most who didn't watch it did not.

Whether the children had seen a Bobo doll before doesn’t change how the groups reacted to the doll after watching the film or not watching film.