Men and women contribute to child rearing in different ways.
Women tend to be more nurturing so their role is disproportionately more important during a child's earlier formative years. They also tend to have a stronger orientation toward domestic affairs. Men, on the other hand, tend to be more strongly oriented toward the public sphere. The home is the focal point. Thus women are at the center of the action, as it were, when it comes to the amount of time they spend with children, while men tend to be more strongly motivated to take care of affairs for the sake of their families outside of the home. The home is also associated with greater safety and comfort, which is something women tend to prefer (we also see this reflected in occupational preferences), while the public sphere contains more risk and discomfort, something men prefer to face esp. when the reward is sufficiently high (this is also reflected in occupational preferences; men tend to prefer taking on more dangerous and higher stress jobs and longer hours in exchange for more pay).
I have used the word "tend" for a reason. These are not two sealed off magisteria. Proportion is probably good way to frame things, though to a point. So men also participate in child rearing, esp. where discipline is concerned (the complement of maternal nurturing is the need for paternal authority which serves the child later in other ways like the ability to relate to authority elsewhere in a healthy way). Women also operate in the public sphere and often also work. Conditions and circumstances can also constrain how male and female roles are expressed.