The best way to make this argument is to draw an analogy with health insurance.
On the one hand, it seems logical that such firms should give extra pay to those workers who do not have children or whose kids do not use the day care that the firm provides. It seems unfair to, in essence, pay one set of workers more simply because they have children who use the company’s day care. But we could also note that there are many ways in which employee compensation and benefits are not exactly fair.
One of these can be seen in health insurance. Many companies subsidize their employees’ health insurance. Should they have to pay extra to those who do not have spouses and dependents whose insurance is also subsidized? If we go even further, should the firms have to pay extra to those who do not use the average amount of health care? Should the firms have to pay men extra, for example, because they do not end up using insurance to pay for prenatal care and for childbirth?
We can also wonder how much we would pay those with no children. Do firms need to look at the average number of kids that employees who use day care have and pay based on that number? Would the firms also need to compensate those employees who do not have as many kids as average? These are very sticky questions that make the whole issue much more complicated.
While it is important to be relatively fair in compensating employees, it is not always possible to be 100% fair. This is the strongest argument against paying extra to those who do not benefit from child care.