You're absolutely right,

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

We think human lives are priceless because we like to believe that everyone is unique. There is no warranty replacement for children born defective; nor could you replace them when they break, go missing, etc.

Everyone is unique, in a sense. The real trip-up in that logic is, just because something is one-of-a-kind doesn't mean it has infinite value.

Looked at from one (unreasonable) level, everything is unique. Looked at from another (unreasonable) level, most humans could be replaced with other humans and very little would change.

There's no obvious-to-me level to look from where this argument applies to humans, but not to pet cats and dogs.

Well obviously everyone is unique in that there's only one of any given life, regardless of whether or not anyone's life is qualitatively unique. But it seems obvious (at least to me) that what's much more relevant is that as self-reflective human beings, we have a responsibility to protect other human lives, within reason. Of course, what "within reason" means is a whole subject on it's own.

That "within reason" is the key.

It would be of massive costs to change your 404, so those missing kids could suck it.

Is the cost (price) of this solution really so high though?

If you're a parent of a kidnapped child who you lost most hope of finding, what effect would seeing this on each and every 404 have? My guess is "not that great", to say the least.

If it's really that much ineffective as some are arguing, I think the costs to our mental health far outweigh the benefits.

You don't have a child, do you? Are you an only child? Would you admit it now if you are?

Why does it matter if he has a child? Because it makes a purely emotional argument less effective on him?

Talking as someone with 5 siblings (4 brothers, 1 sister), I find that one brother is generally as good as another whereas my sister is unique.

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.