With all due respect, Ramesh, using public policy to create incentives for behavior believed to be socially desirable is something close to the definition of social engineering.
As for your broader case — that child tax credits offset the fact by having children parents do more to support Social Security and other entitlements through the creation of future taxpayers — it’s never made much sense to me. Parents do create future contributors to Social Security et al., but they also create new beneficiaries. Given the upside-down finances of those programs, every new contributor-beneficiary is a net loss. So, if we were arranging taxes based on evening that out, we would penalize parents rather than subsidize them, which I think illustrates the folly of thinking about parenthood as a federal cash-flow question.