What happens if she leaves?

Should she be able to come back and take the kids? Illinois says maybe not. If the mother is not fit or not interested, the father should get custody:

The Court stressed, "the parent-child relationship is an important interest that undeniably warrants deference and, absent a powerful countervailing interest, protection." A parent's interest in the companionship, care, custody and management of his or her children rises to a constitutionally secured right, given the centrality of family life as the focus for personal meaning and responsibility.

A child has an equal right to be raised by the father, and must be awarded to the father if he is the better parent, or mother is not interested (United).

What I find most interesting is that this case took place in 1972. It seems as though this case should have set the wheels in motion for fathers all over the country. Unfortunately, this type of action is not taken very often, even to this day.