While A Little Princess uses a very omniscient voice, it's also the case that the majority of the novel works through Sara's point of view and her experiences. So it's a bit of a break with the flow for Chapter 14 (What Melchisedec Heard and Saw) to stand entirely apart from her. It occurs to me, though, that in a way, Melchisedec the rat is standing in for Sara's connection to the events.
For the most part, we can view the anthropomorphism of Melchisedec as part of Sara's fanciful invention. (We can also allow of a bit of authorial ignorance regarding the social biology of rats, in positing Melchisedec as the head of a cozy nuclear family, with Mrs. Melchisedec waiting at home with the children.) The title of the chapter is not the only prompt we are given that the rat is to be interpreted as our window into these events. We view the intrusion of Ram Dass and of Mr. Carrisford's secretary into Sara's attic through the rat's eyes and reactions, even as the authorial voice assures us that, "Melchisedec did not know [who they were]." And it is implied that we are made privy to their discussions there through Melchisedec's perceptions, despite acknowledgement of the rat's deficiencies as a witness. "How much he understood of the talk he heard I am not in the least able to say; but, even if he had understood it all, he would probably have remained greatly mystified."
This leaves us with a somewhat curious mystery as to why Melchisedec has been set up as our witness at all. (And that's aside from the question of whether a proper English rat would have been expected to understand a word of Hindi which--as discussed in the consideration of Ram Dass's linguistic competencies--must be assumed to be the language in which he and the secretary are conversing.)
Digression: I'm always fascinated by such narrative structures for explaining or excusing how the content of a story is transmitted to the reader. During the Worldcon panel on Shelly and Austen, I came up with a shorthand for this fascination: the fiction of a story as truth versus the truth of a story as fiction. That is, has the author created structures to create the illusion that the events of the story actually happened in real life and in real space (somewhere) and that the knowledge of those events has been conveyed via a documented "chain of evidence" such as first-hand accounts, letters, diaries, etc.? Or is the structure of the story such that the author and reader begin with an understanding that the events are entirely fictional invention, and that therefore there is no need to explain how the author became familiar with them? For example the "fiction of truth" approach, when applied to secondary worlds, requires a traveler's tale such as we see in the opening of Edgar Rice Burroughs' Barsoom novels. The topic came up in the panel discussion in the context of the framing narrative for Frankenstein. In contrast, the "truth of fiction" approach is the default today, where no explanation or excuse is considered necessary for relating stories with no chain of transmission from the characters to the reader.
In this context, the use of Melchisedec as part of the "chain of narrative transmission" is both nonsensical and unnecessary, given that other scenes external to Sara's direct experience have been related directly and given that there is no fiction that the rat in any way conveys the knowledge of these events to another party. And yet there's clearly a sense that he is standing in for the reader's access to the events of this chapter in some fashion. But I digress...
The purpose of this chapter is for Ram Dass and the secretary to discuss the state of Sara's attic, the plans and mechanisms for how to transform it into her imagined vision of the cozy and comfortable space it could become, and a discussion of the practical logistics of how this will be accomplished in secret while she is sleeping. One thing I like about this chapter is that it shows how Ram Dass has become personally emotionally attached to Sara and how the transformation began as his idea, suggested and elaborated by him to Mr. Carrisford. I like this not only because it gives Ram Dass significant agency in the outcome of the story, but because it suggests a personal motivation beyond the superficial suggestion of a reflexive desire to serve the little girl who "has the bearing of a child who is of the blood of kings". On the occasion when Sara first meets Ram Dass, she considers that he--like she herself--might be feeling lonely and homesick in this land far from their common origins. And this chapter provides confirmation that this evaluation was correct ("I am fond of this child; we are both lonely.") and that Ram Dass's affection is based in part on this sense of connection.
From a more practical point of view, our eavesdropping on Ram Dass and the secretary turns what would otherwise have been a mystery (and still is, to Sara) to a conspiracy between the reader and Sara's benefactors. It also softens the reader's empathetic misery in the following chapter when Sara experiences a roller-coaster of emotions, because we know about The Magic that's about to appear in her life.