Why do people call the most dangerous spider child of earth?

Answer 1

Because people are stupid, though in this case it's hard to tell exactly where the stupidity comes in.

Somebody somewhere has used just about any name you can think of for something or other. Unfortunately, even when they use the same name, they're often not talking about the same thing. For example, the term "daddy long-legs" can refer to any of several different kinds of arthropods and at least one plant. This is why biologists tend to prefer "scientific names", where you can be pretty sure you're referring to the same organism.

If you Google for "child of earth", most of the references seem to be to wind scorpions/camel Spiders, which are neither scorpions nor true spiders nor particularly dangerous, since they're not venomous. They're slightly more closely related to scorpions than they are to the true spiders, though they look more like spiders than like scorpions.

The most dangerous spider is debatable, but a strong candidate is the Brazilian Wandering Spider, which is venomous, aggressive, and large enough that its bite stands a reasonable chance of penetrating human skin (there are disagreements about how dangerous its venom is, but there are reliably attested cases of human death from bites). I can't find any evidence that anyone calls them "child of earth", but vide supra; it's certainly possible that somebody does so.