You may be over reacting by having a do not feed sign on your dog. We have a huge festival in Birmingham every year called Do Dah day. In all the years I have been going I have never had anyone try to feed anything to my dog. There are many food suppliers with regular kibble and treats but none were ever offered to my dog directly. We had many vendors offer food and treat samples to us but never directly to our pet.

Unless the people of Canada are very different that those of the southern US then most people do not do anything to your pet without your permission. That goes for petting or feeding. Now the child will rush up and start petting without asking but if there was a sign saying do not pet they would not read it.

In Alabama when we go to the bank or Starbucks we are always given a treat but it is always given to the adult and not the dog.

If your dog has problems with other dogs sniffing then the type of festival you are going to is not the best place to socialize your pet. I can assure you that there is a lot of sniffing going on when you have thousands of dogs all walking around and greeting each other. if you have reservation as to whether it will tolerate sniffing well then I recommend you stay at home.