Dear Babe, that sleepest cradled by my side, Whose gentle breathings, heard in this deep calm, Fill up the intersperséd vacancies And momentary pauses of the thought!
Coleridge, as he pauses his thinking, becomes aware of his baby's breathing. This changes his train of thought, again (which is why we're in a new stanza).
Now, he stops addressing the reader (this isn't really a "conversation," it turns out—it's more of a monologue) and starts speaking to his baby.
My babe so beautiful! it thrills my heart With tender gladness, thus to look at thee, And think that thou shalt learn far other lore, And in far other scenes!
Coleridge is going to make sure that his son will enjoy a life lived close to Nature, not one stuck in a school like Christ's Hospital in London, where Coleridge went, and where the headmaster loved beating kids. (Christ's Hospital still exists, by the way, though it isn't full of sadistic teachers with the license to hit you, anymore.)
He's excited, glad that his baby will lead a more natural and peaceful kind of life. He'll learn "far other lore" from "far other scenes"—since Nature teaches different lessons than the city.
But what are those lessons? It looks like Coleridge is about to explain…
There's some alliteration here too, with all the Th sounds in "thus to look at thee/ And think that thou." Check out "Sound Check" for more.
[…] For I was reared In the great city, pent 'mid cloisters dim, And saw nought lovely but the sky and stars.
Coleridge further explains what he implied in the last set of lines: he was stuck in the dim "cloisters" (confines) of an urban school, and the only association he had with nature was through the sky. Everything on earth had been built over by humanity. Hopefully, he's saying, his son will have a much different experience…
But thou, my babe! shalt wander like a breeze By lakes and sandy shores, beneath the crags Of ancient mountain, and beneath the clouds, Which image in their bulk both lakes and shores And mountain crags:
Coleridge predicts that his son will have the freedom to wander in Nature and learn from it.
"Wander like a breeze" is a pretty simple simile—like a breeze, his son Hartley will go wherever he feels like going.
The clouds in the sky, according to Coleridge, mirror the shapes of the things on earth ("image in their bulk")—like mountains, lakes, and shores.
This fits with the use of reflection imagery in the poem. The same way the fluttering film reflects the thoughts moving in Coleridge's mind (which themselves reflect the Spirit), so the clouds reflect the earth.
[…] so shalt thou see and hear The lovely shapes and sounds intelligible Of that eternal language, which thy God Utters, who from eternity doth teach Himself in all, and all things in himself.
This continues the reflection image from the last lines, in a way. Just as the shapes of the clouds reflect shapes on earth, Coleridge's son will come to see that Nature reflects God. Its shapes and sounds are like God's "eternal language"—they testify to him and reveal his existence. (So maybe God was the "stranger" whom Coleridge couldn't find in the city, and whom the church bells prophesied.)
God uses all things to reveal himself, since he contains all things within himself, as well. This is sort of pantheistic (pantheism is the belief that God is present in everything, or even is everything).
The structure "Himself in all, and all things in himself" is called a chiasmus. It's made of two statements, which invert each other. (For another example: "The squirrel was in the hamster's heart, and the hamster was in the squirrel's heart.")
Great universal Teacher! he shall mould Thy spirit, and by giving make it ask.
Since God instructs humanity through Nature, Coleridge calls him a "Great universal Teacher"—"universal" in the sense that God instructs everybody. He's not leaving anyone out.
God, according to Coleridge, will mold little Hartley's spirit by giving him the gift of Nature, since it shows that there's a greater creative process behind everything. By giving Coleridge's baby this gift, God will make Hartley "ask"—ask to know God, or enter into a closer relationship with him.