I'm not saying these symbols qualify as a language, but this argument against it does not hold up.
There is no such thing as a child developing "independently" in the sense that you are using it here, and anyone who would try would be rightfully sued for child abuse in most countries.
Even a child with prelingual deafness who is denied Sign language and access to Deaf culture (and I recommend looking up what terrible impact that has on such a child's development and quality of life), is still surrounded by human symbols from the day they open their eyes.