I think what more often happens is one of the parents are almost entirely responsible for the project, and the kid understands it well but had little to do with its creation/invention/discovery. If not, then it's quite a coincidence that most of the kids in these stories just happen to have a parent who works in the same field their project was exploring.
I don't understand, why can't the parents be helping and mentoring their kids, without necessarily doing the work for them?
I'm a son of two voice actors, and have done some voice acting myself as a 12-year-old. No, it wasn't a coincidence, but I also guarantee you that my parents didn't act on my behalf! I just, you know, learned from them.