So, first, the "cobra effect," like most economic proverbs, is apocryphal. It didn't happen. It's a story economists tell their kids to keep them from sharing.
Second, there's a big damn difference between "I'm going to grow a snake because I'm getting paid for it" and "I'm going to get my kids kidnapped so I can collect a tax credit without actually having to pay for the kids." It may not appear that way if one's working model of people is "dispassionate rational actor," but it turns out in the real world most people have some kind of irrational emotional attachment to their kids.
Finally, I'd submit that "Cobra Effect"-type problems arise as a factor of two errors - the first is misunderstanding the cultural group for which you're designing practices, and the second is a weirdly inflexible view of society that accepts someone manipulating the system as just a sort of thing that happens and not an egregious breach of the social contract warranting a response.