Me Myself
Back to my username
How about "the Earth is round and here's a test to confirm it"?
Beyond simple stuff that a kid has to learn very early (e.g. "don't touch the stove"), I fail to see why you'd have to teach much of anything to a child uncritically.
The whys always have an end. Ultimately, s/he will have to trust your information and will do because you are his father (more likely than not)
"What does this mean in this test?" "Iteans this and this" " how do I know this is true?" "Because this and this organizations made those observations" "how do I know those are the organizations that made those observations?" "Bcause you can see it on this webpage" "how do I know this webpage is not a fake?"
Etc etc
There comes a point where there are no reasonable directly provable whys left.