In my kidsstore, the most popular fabrics include for boys; robots (my biggest seller), trucks, race cars, trains (Thomas), tractors, skateboards, snowboards, camoflage, skulls, jungle animals, and Legos. Apparently the most popular superheros are the standy by Batman and Spiderman plus any of the Avengers.
Girls seem to like princesses (my second largest seller), tiaras, tutus, ballet shoes, horses, unicorns, owls, cats, penquins, turtles, tree frogs, jungle cats, cheer leading, gymnastics, hearts, peace signs, rainbows, and cell phones (with words like LOL and BFF). Girls are also into things mystical and supernatural.
I've had moms decide on one thing, but the kids went with something totally different. I try to gear my fabric choices to the things the kids want. About half of my customers opt for things licensed, but the other half do not their kids focused on so much commercialism.