I don't think tax relief for having children would be good, however, tax relief through through income splitting would go along way to helping out families.
Normally, a household of two people, who both make $75,000 a year (so that's $150,000 for the household), for example, would pay less than a household family of four with a single income earner making $150,000 because they're paying a lower income bracket individually. Though the household has the same amount of income, the family where one parent is staying home to look after the kids pays more tax. Following countries like France, where tax is based per household than per person, I think that's something that could really help out families.
Anyway, my thoughts on the matter are that people should be smart and responsible about when, how many, and why they're having kids and put consideration into how you would raise (and afford to raise) them. Unfortunately, you cannot police intelligence - nor should you.