You jest, but "5 whys" is a real tool: https://en.wikipedia.org/wiki/5_Whys

Some honesty here would help: "I/We don't know." "This is what works." "Enough people just decided on it."

I think it would make a big difference to the child's worldview, that we as a species are still pretty much new to figuring everything out, and that adults aren't infallible know-it-alls, not to be questioned.

Cultivating "doubt without helplessness" may actually turn out to be beneficial to overall society.

There is a variation:

- Why?

- [Explanation] Why did you want to know?

- You are not allowed to ask questions. [Asks different question]