dina, i know this is late, but here's an article that'll help explain the type of mold that infects berries, for future reference.
Botrytis Fruit Rot "Gray Mold" of Strawberry, Raspberry and Blackberry, HYG-3017-94
it's a little advanced for a kid, but it might help you help her study.
it seems that the mold spores infect the blossoms, lay dormant, and begin to grow as the fruit ripens, especially under low light and moist conditions.
i'm guessing that raspberries either ripen more quickly, or have more available moisture.
i get this mold on my raspberries, and occasionally on my strawberries, when i used to grow them.
i've never seen it on blackberries.