The reach your hand out so they can sniff is the most common thing you see people do. In fact I do it too! But I'd never approach a dog without asking the owner if it's okay in the first place. At the dog park I don't approach dogs. I let them approach me then I'll pet them if they want it.

I've come across children who are obviously taught puppy manners by their parents. They ask if they can pet the dog first. That's the BEST thing to teach a child, along with the above items.