How to Teach Students to Not Guess the Teacher’s Password?

By making it impossible to guess!

When I was in school, questions were 100% memorization. You were supposed to remember the answer, not understand it. Math required calculation, but that just meant remembering the algorithm of calculating. I never needed to think about how to solve a problem, just to recognize it as one of a very few known types and remember the method that was taught to solve it. (If I used a different method to get the right answer I would get points deducted.)

If you ask questions that can't be answered from a repertoire of remembered facts and require understanding and thinking, you will force your students to think for themselves.

Of course you will probably also cause a student and parent revolt against unorthodox teaching methods that make class harder than it has to be.

Young children often do not the basics

You're missing a word here.

If it's meant to read "do not know" (or understand, etc), it's a true and unremarkable statement. But I suggest that it's also true that they do not get taught the basics - if by "basics" you mean the fundamental concepts of a field. In third grade, children aren't capable of really understanding most of them.

The basics of math are number and set theory, formal logic, etc. The basics of science is epistemology and the scientific method. The basics of biology and physics are perhaps the Modern Synthesis and the Standard Model. The basics of history is... a good overview of world history, I guess. The basics of religion are a whole lot of complex social, behavioral, and historical sciences.

These mostly aren't things third graders can understand. But most things they are taught really have very complex explanations. It's still better than nothing to teach them the what without the how and why; the results of science without the methods. The same applies to more advanced concepts taught at each later grade.