A baby's butt is defined by two methods. The first method is a woman doggystyle with her stomach hanging down. Her stomach appears to be split down the middle, hence two small but cheeks with a crack resembling a baby's butt. The other method is a male with a beer belly who can push his stomach together and create two cheeks with a crack down the middle.
Brenda's baby's butt was shaking all over the place when I was hittin it from behind.