Ah, one of my favorite arguments. The good 'ol, "there is no price which is too high to save a life (period, full-stop)".
Do you actually believe that? Really? Or am I just reading you wrong here?
After all, why shall we even try to save human lives?
According to the current population growth rate[1], we have a relatively high supply of humans on earth, so anyone dying or disappearing will most definitely be replaced within a short period of time. Merge that data with the kidnapping statistics[2] and you'll see there's nothing to worry about.
Moreover, a child had likely not accomplished much in his life, so his disappearance could only affect a limited number of people.
[1]: http://en.wikipedia.org/wiki/Population_growth [2]: http://en.wikipedia.org/wiki/Kidnapping#Statistics
There's no obvious-to-me level to look from where this argument applies to humans, but not to pet cats and dogs.
If it's really that much ineffective as some are arguing, I think the costs to our mental health far outweigh the benefits.
Based on this, your question of "Are you an only child?" seems only to be helpful to your argument when used against people with a relatively small amount of siblings of a particular gender. This condition is, let's be honest, a relatively new development in the first world and simply doesn't generally exist in the developing world.