I partly agree with Ramesh when he writes about single-earner vs. two-earner families:
What if the single-earner couple is taking care of an elderly relative? Should we create special tax rules to take account of this situation?
It seems to me that there is at least as good a case for that as there is for the child tax credits Ramesh endorses. We could take that as an argument for elder-care tax credits, or we could take it as an argument against trying to create “a tax system that achieves fairness defined by taking account of every possible cost difference,” with the understanding that one of those possible differences is the cost of raising children. There are costs and benefits to having children, both of which vary from child to child, but I do not see any particularly compelling case for making that a question of federal social engineering.