Trust is not an assumption. My 3yr old son trusts me because I have never lied to him. Since I have never lied to him, why would he ever assume I'm telling the truth? Only when someone does lie to him and he finds out the truth, does he begin assuming others might actually be lying to him. He doesn't assume I'm telling the truth because he believes I'm telling the truth, no assumption necessary.
We do not start off assuming things, we start off believing things. Assumptions come later, after we've been lied to.
First, using children in epistemology is usually not a good sign. What is good for children to do epistemically is not good for us. He may not have a reason to do things like this, but he has probably not have had knowledge of people lying to him. My point is exactly what you said, he might have no reason right now to question his beliefs.
However, my entire argument is that we, rational adults aware of philosophy and discussions of skepticism and the limits of knowledge, do not have that luxury. No matter how hard I try, I can no longer ignore the possibility that my perceptions are being lied to and that I am actually a brain in a vat. Assumptions come when we start to realize that we are fallible; that we are prone to error and that we can make mistakes. Once when your son grows older and discovers people lie, he can no longer rationally believe every single person he meets will tell him the truth. If he did, he would be naive and foolish. Therefore, he has to assume others are telling the truth. He may not consciously do it; often, we don't consciously to decide at all. However, when I play the skeptic and push him against the intellectual wall, he must admit that there is the possibility that anyone he knows is secretly lying to him; this possible world might be so far-off that it seems weird to even consider it, but the world is still a possible one. He must acknowledge that there is a chance, no matter how small, that the possible world is the actual one. He is rationally justified in making this assumption; so rational, in fact, that we consider it epistemically obligatory that he do it in some cases. But there will always be the assumption, no matter how well justified and rational, when it comes to the limits of our knowledge.
Beliefs do not influence reality. If someone believes in God, what does it matter for the actual existence of God? If someone does not believe in God, what does it matter for the actual existence of God? Merely because you believe me to be a conscious human being and I claim to be one does not make me one; I still might be a philosophical zombie. To use substitution to show the absurdity of this position, a member of the Peoples Temple might have believed Jim Jones to be a proper religious figure, and Jim Jones claimed to be a proper religious figure. However, this alone does not indicate that it is true that Jim Jones is a proper religious figure.
Beliefs alone do not generate knowledge. Justification does. And no system of justification is without its limitations.