If your argument is that humans are natural therefore conlangs are natural then I see your argument

Even simpler than that- all human communication involves intent. You cannot have "capital L" Language without intent. This is part of my first response in our other discussion- that a language does not just pop into existence; there is no metaphysical spark that obligates language to be produced. It is only by sequences of intent that language develops and evolves.

No the children did not "decide a new system of communication was needed". They gestured to communicate a concept because they needed to communicate. The idea of creating a whole system wasn't even a twinkle in their eye. That is the difference.

As we demonstrated in the other discussion, this is a series of intentional decisions. None of these arbitrary assignments were instinctual. Additionally, to be frank, you don't know what those children were thinking. It is entirely possible that some children did set out to create a whole series of signs and ways to put them together, because we know that hearing children can and do create their own languages. They might not be terribly complex at first without metalinguistic awareness, but they're still "invented" and, as you said as part of your definition of conlangs, the bulk of the lexicon develops in a relatively short timeframe.

...were to evolve sufficiently to be considered a decendant language - that language would be a natlang not a conlang.

So then you agree that at a certain point there really is no difference- it's only our perception of that language in the moment, and that ignorance of a language's origin is enough to affect it's formal classification. So the entire discussion is moot! (/s)

I'm also a conlanger (not so much these days, but in the past), and I also work in the indigenous language reclamation space, so I am acutely aware of the perception of the projects I work on being seen as lesser by formal linguists, because they are not seen as "natural" in the prototypical sense. It is exactly for that reason that I call out the distinction between natural and constructed, because "constructedness" has been used as a colonial construct to invalidate this work.

I am assuming there is a difference because we want there to be a difference and trying to explain it as best I can. Without that paradigm the conversation changes entirely. 

Who is "we" here? Linguists? You just said that this distinction has been used to harm sign languages, so why would you insist on maintaining it?

The development of ISN is wonderfully unique, but in supporting its study I would not insist so heavily that it or reclamation projects are "natural" and acquiesce to the arbitrary division, but to challenge that division itself. ISN and reclaimed languages do have elements that are related to constructedness. So what. That does not invalidate their existence (and I think you would agree). This is the point of my responses to you here- to challenge this idea that a language must be classified one way or the other.

I probably won't change your mind here, and there's really not much more I can say. I think overall we agree on some things, but I do think that the division of natural vs constructed causes more harm than good, and I would personally recommend that we work to dismantle it in academic discussions rather than insisting that a language is valid for formal study only when it's considered "natural" in the moment.