Why exactly do we need to change the rules? Are people forbidden from strapping their child into a car seat (they may be, I don't know). If they are allowed to do so, those that value the extra safety will do so. I understand that there may be some sort of negative externality here: a flying child could hit and injure someone else, but there are far more laptops and dvd players on a flight.

I don't really feel like I want to FAA telling some parent how to travel with their child if there is no effect on me whether they lock them down or not.

The other problem with passing the rule is that it will raise the cost of flying. Since flying parents with kids will be much more price sensitive, the airline would be smart to cross-subsidize that cost to more inelastic travelers, thus also raising ticket prices for those traveling without kids.