Q. My Harlequin Great Dane is fine on the leash until we approach other dogs. The hair on his back stands up and he aggressively lunges forward at other dogs. He also is afraid of children. Is there a way to train him to stop displaying such aggression?
A. OK, my flags are up. This size dog has the physical ability to take you where you do not want to go. To begin with, I assume that he is neutered or will be promptly. This is not optional.
Your Dane does not have to like other dogs but he cannot decide to attack them. This is where your foundation training becomes critical. Because of his size and the problem, I strongly suggest you get hands on help from a local qualified professional. This is not a "do-it-yourself" project.
Also, please consider using a head halter on the dog, as this should give you better control.
Use of food reward when another dog shows up will create a positive connection versus aggression The goal here is to create a positive (treat) when another dog is in the area½dog, treat, good stuff. But again, basic control is a must.
As for his fear of children, if he was not exposed to children as a puppy he may have no idea what a child is. This situation can usually be helped but how much progress can be made can't be said. I refer you to our book "Childproofing Your Dog" by Kilcommons and Wilson for detailed suggestions for this upsetting problem.
You are doing the right thing by seeking help.