Children need to know that if a dog's owner says 'Yes' then they should only touch a dog on the back and sides, as shown in green in the above poster. Children wouldn't like it if someone put their hands in their face or messed up their hair; dogs don't like having strangers anywhere near their head either.
It used to be that we taught children to touch a dog under the chin or on the chest, however, studies have been done that show this is actually very stressful for the dog and even if you think about it logically: if the child is leaning down to touch the dog under the chin their face will be close to the dog's face, so if the dog does react where is it going to bite? The face! You wouldn't stand your child behind a horse so why put it at the pointy end of the dog.