If you want to argue against anecdote, it's best to provide actual facts instead of your own questions. For example, only 1 in 5 drowning deaths are children, and there's no reason to assume a significant portion of those are unsupervised [1]. Should we be helicopter-observing adults instead, or do their lives matter less? Traffic accidents are the largest cumulative killer of children [2]. If you want to go the route of "how many aren't here to tell us about it" you should really be arguing that parents should avoid putting kids in or near cars as much as possible.
Children die, and that sucks a lot, but if you start making emotional appeals based on preventing all possible child deaths you're going to end up espousing that we keep all kids in padded rooms until they're adults.
[1] http://www.cdc.gov/homeandrecreationalsafety/water-safety/wa...
[2] https://www.cdc.gov/injury/wisqars/pdf/leading_causes_of_inj...