Uh. I think I have to side with the pet store here. It can be really tough to sell an animal to someone who is going to do something that is known to be detrimental to it. Goldfish (fish PERIOD) should never, ever ever live in bowls however tiny they are. There's no room for cover, no filtration or anything. I also have to wonder how old the granddaughter is because what if said granddaughter refuses to take care of the goldfish?
How does the pet store know Grandma really will buy a bigger tank? And if she knew what she was doing, she would know that medium to larger tanks are often better for beginners because they are more stable. Granted, for a little kid, I'd say 5-10 gallons and a hardy fish or two would let them have say, a betta or something shiny while giving them enough room to decorate rather than 'here's your fish in a bowl'. You can observe more behaviors, enjoy the decor... and if you miss a water change or two, it wouldn't be the end of the tank. Perfect for lil' Susie and Grandma in case they get sick/go out for a week/whatever. And frankly, I have yet to see an animal that is happy with nothing in its environment. No cover, no stimulation... what a miserable existence.
So grandma, I have to say that you are in the wrong. If you truly want your granddaughter to get into fish keeping, let her say when. Pets are rarely good gifts. They need research, preparation and supplies. Let your granddaughter pick the tank and fish instead of all but insisting she needs to start with a bowl and a goldfish. Frankly, I wish they would stop selling those damn bowls or mini novelty tanks. They're cruel. But mostly, I feel that children should be allowed to keep animals when they are old enough to understand (And there's nothing to say mom/dad/grandparents can't help. They -should-) and certainly, they should have a say in what pet they get.