The Sioux City accident is the only high profile incident where lap children were indeed killed due to lack of child restraint systems. However, as another poster stated, there have been numerous incidents where children have been injured when parents couldn't hold onto them in turbulence. Most people associate turbulence with just being bounced around a little bit, but for those of us that fly on a regular basis and in and out of different geographic regions we know how violent turbulence can be. I have the battle wounds to prove it! I have seen many people with infant children fall asleep on flights and when unexpected turbulence hits their arms are lax enough that the kids can literally fall on the floor. I've also seen people make their children hold onto infants and that is a big no no, kids don't have the instinct or strength to hold onto an infant in certain situations.
Just my opinion, but after seeing first hand and hearing about other incidents from colleagues what can happen to "babes in arms" I think car seats should be mandated.