I think that's almost certainly true (assuming the lap child is not otherwise secured with a harness-style device like the Baby B'Air).

But as it pertains to the safety of passengers other than the child, is it not also true of personal items and would-be projectiles like laptop computers?

And how do you define "impact"? A collision involving an airplane at taxiing speed might present the very rare instance where securing a child could pay safety dividends, but a mid-air collision or any sort of crash where the plane was at airspeed prior to the impact undoubtedly would not except in the one-in-a-trillion miracle situation.