kids learn, walking talking reading arithmetic and physics, by doing things in the physical world. Adults may speak differently to kids than adults, but it's a stretch to say it's synthetic. Equivalent to synthetic would be a group of kids that just grew up together and made up a novel language.

granted synthetic is closely related to synthesis, but in common parlance synthetic would mean something that is not natural or abiotic in some sense, in case of synthetic data, it should imply data that doesn't occur naturally from human and natural sources. i.e. synthetic data would be exactly the one that is assembled from algorithmic output. granted im not able to explain it as well as i understand it.