Isolating children so that they would speak in
the original language is a bit like closing your eyes and throwing paint at a canvas in the hope of painting
the original picture: There is no such thing, and even if there was, you wouldn't find it that way.
Languages change, diverge, merge. We are capable of speaking in many different ways, and none of them are the only right one. Each pair of children in the experiment would invent a new language, albeit a severely stunted one.
We have an innate ability to create language, but the kind of language that we create is shaped by external factors.
It is a tautology of evolution that human beings have been talking as long as they have been able to talk, and have able to talk ever since they started doing it.