From what you write here, I'm not sure. I'm sure that what I call internal assent is not the same as a belief that I have a belief, but I'm not sure whether that's because we differ on the nature of internal assent or whether we differ on the nature of belief. Rather there is a readiness to report what one believes. If someone's talking and you nod your head in assent, that isn't in reaction to an introspection. Or at least it does in my case.
Children can agree with, and believe, something without being aware that they believe it.
So my response to this is to deny that assent is but another belief. They are two very different things. I would be very surprised. In order to have beliefs about their beliefs, chimps would have to have a notion of what a belief was. I think that's far too abstract a notion for a chimp.