First I'll cast my own definitions; Metaphysics is the science that tries to answer the question of what is possible for us to know? (I use the verb "to know" in the sense of Aristotle's Logic and Galileo's scientific method). Epistemology is the science that attempts to answer the question of how do you know what you know?

Metaphysics must begin with one of two assumptions, viz. either the universe we sense is the totality of all that exists or can exist, or, there are dimensions, Plato's forms or Heaven for example, that are beyond our senses.

If you take the former as true, then the first task of Epistemology is to prove that our senses are a valid method of apprehending reality. If you accept the latter assumption, then you reduce yourself to subjective assertions which cannot be proven.

Finally, as regards children, Ayn Rand wrote a series of articles titled The Comprachicos in The Objectivist August – December 1970 in which she gives an excellent exposition of the intellectual development of children. Have you read these?

A is A
Every logical system has to start with an observation or assumption. Science and Rand start with observation and they check their system against reality, ie observation, just as in physics. The idea that you can trust your perceptions, not that they are infallible, you build logical systems and you check these systems against reality, is what every advance in this world that humans have created is based on. The evidence is overwhelming, that this is a true metaphysical and epistemological system.
In math, that fundamental assumption is axiomatic. In physics and Rand, it is observation. A is A, therefore you can trust your senses. You can trust them, however, they are not infallible.