Anyway, you said all the children lost their memories.

Isn't that unusual? Something must've been done to them...

...They don't remember the details, but they're aware of being somewhere else before coming to the facility.

It's unlikely they were born in the facility.

If that's the case, then the children were born somewhere else and were taken, but...... if they were about 4 years old at the time... it would be strange for them to not know where they came from.

In other words,

...Are you really gonna say we were drugged again? I just told you nothing like that happened.

What if your memories of being drugged... were erased by the drug...?!

Huh? Can they do that?

...Maybe! Their true purpose might be to conduct clinical research they can't do in the surface world!

They sell the strong children as assassins to Yakuza to dispose of the test subjects they're finished with and fund their research!