While it seems logical at first, when you consider it seriously it quickly falls apart.

Even not considering such questions as “How do you enforce it?” (What are you going to do? Take the kid from them? That would quickly turn into a giant mess) or “How effective in actuality it is going to be?” (What stopping someone malicious to ace the exam and then abuse their kid without legal repercussions?) much simpler reason is that it would even further hinder the already falling birth rates. And most of the goverments and people don’t want that.