The fairest and most effective counterclaim is: "Saturday is the most dangerous day of the week in terms of traffic accidents and other pedestrian hazards, so such a move would be less safe for children."
This counterclaim directly addresses the argument's focus on safety by presenting evidence that moving Halloween to a Saturday could potentially increase risks instead of mitigating them. It highlights a significant concern regarding public safety in relation to the proposed change, making it a strong counterpoint to the original argument.