do not say "Oh it's alright sweetie, you don't have to ask. S/he loves kids" when I am clearly telling her "Dd2 ask first" or "Dd2 do not disturb that doggy or I will take you home" because the next dog they meet might not love kids.
Dd2 no longer asks before running off to meet a dog, because she has heard this so many times.
This a big problem to me. I can no longer walk my dogs on the school run because it is prime dog walking time and I cannot walk my dogs, aid The Devil dog with his socialistion and fetch dd2 back from the dog and child aggressive Collie she is trying to pat because the collie owner further down the park told her all dogs love kids 
I am considering leashing dd2 for our next walk.
I understand that some people like children petting their dogs and owners of larger breeds or 'dangerous' breeds might find it refreshing to meet a child who is not fearful of their dog and a parent who is not hysterical, you can let her pat your dog. I don't mind that. Just make sure she asks first. And make sure she asks everytime, even if Jake the GSD is her 'friend dog' because Elvis her 'friend' the Boxer only likes her petting him when his tummy isn't poorly.
It's no use me telling her one thing and 90% of dog owners telling her differently.