From your post there can i ask is he acting aggressive because hes anxious or because he genuinely hates other dogs? is he actually going over and grabbing hold or is he just pulling and barking whilst you hold him on the lead? Have you let him off the lead near any dogs, what did he do? If its due to anxiety, a nervous dog is a dangerous dog, especially round kids, and i think smacking the dog wont correct it. If its acting aggressive out of hate for other dogs, id say it would be easier to correct and a good whip will give him boundaries.