The terms "Mama's Boys" and Daddy's girls" can be found to be precise when applied to certain children and even adults. It maybe dependent on the person and how he or she was raised if these terms continue to be true in adulthood. Some parents encourage the dependence of their children on the adult to an unhealthy degree even into adulthood and make these terms frighteningly fitting.
As someone who has raised both a young man and a young woman, I can say with some certainty that men tend to be raised by their mothers, while women are brought up by their fathers. It seems as though the opposite genders serve to create a stronger bond between the two.
In general, the terms are not exactly derogatory in nature. They simply point out a fact about someone. So, if someone has an unusually high proclivity for their mother or father, then this is what they are called. Certainly, it's not a negative thing to most people. A son who loves his mother is something that should be revered, and the same goes for a daughter who looks up to her father. But, yes, they are usually correct.
They are generally precise, even when used as pejoratives. A mama's boy or a daddy's girl is a child who is especially attached to the respective parents. This could mean they have a great love and reverence for their moms or dads. When used as insults, these terms can be accurate. A mama's boy, when applied to a man, is someone who takes orders from his mother or still lives in her basement. A daddy's girl is a woman who is looking for a man who reminds her of her own father.
No, they are not technically precise. Technically, children belong to their mother and their father, so they would be both "mama's boys" and "daddy's boys". On the other hand, the term is sometimes used loosely because it may seem that a child is more attached to one parent than the other but that doesn't mean the child doesn't love the other parent just as much. I do understand the terms though, I am just pointing out it not being precise.