The reason people receive tax benefits for having kids is because the cost of raising a child to age 18 in lets say for example, the United States, is estimated to be approximately 245 thousand dollars or around 220,050 euros.
There is no unfairness considering both people will come out equal as the person without children isn't having to support an economic parasite for 18 years. In fact, its a well known fact that the person without kids and no tax benefits will still have much more wealth than those with both.
Also if your argument is based on fairness, isn't it inherently unfair that without tax benefits and other federal programs to aid parents raising children for lower income households that it would be essentially impossible in developed nations?
http://money.cnn.com/2014/08/18/pf/child-cost/
Economically denying someone the right to have children is far more unfair than denying someone who will come out richer anyways the right to have a tax break they dont need.