Why is it less of a crime to murder a child than to murder an adult?

Just reading a story about a 17-month-old baby found in the Berkeley Marina. He was killed by his mother's boyfriend, who was a convicted child killer. The question was whether she knew he was a convicted child killer and why did he only get 11 years for beating a 3 year old to death back in the early 90's, which is why he was able to kill again. He killed the baby, and dumped the body in the marina, and then killed the mother because she witnessed him killing her baby. So why is it less of a crime to murder a child than to murder an adult? People who abuse and kill children get less time than someone who killed an adult? What is wrong with the system, are children looked upon as less of a human?