"Because kids do better with religion" is a silly and shallow answer. My reason? Because in the beginning, God created the heavens and the earth. It's true, and it would be immoral to teach my children anything but the truth, or even to fail to teach them the truth. In other words, the "principled urge to keep the faith" that is dismissed in passing is both the real reason and the best of all possible reasons.
Not necessarily silly and shallow per se, but it is utilitarian. It demonstrates the lack of commitment to Christ on the part of the parents, because if you know Christ and are committed to Him in all things, the better answer would be, "I cannot not teach them, any more than I could not breathe, or my heart not beat, or my lips not taste water." You could then quote St. Patrick's prayer ("I bind unto myself today").