What do you call a break in rhyming in the middle of a book / poem for dramatic effect?

Question

I'm creating a children's book for a school project. I've been tasked to identify some literary devices used throughout the book.

There is a section of the book which I want to highlight because it represents a major event. There's rhyming throughout the book. However, when the major event happens, the rhyming suddenly stops to produce a dramatic and eerie effect. I'm having trouble identifying what exactly this is.

What would you call that break in rhyming? Would it be considered a literary device?

...

They built houses and cities, what a marvellous sight.
Oh how amazing it was, it just felt so right.

---

*Rhyming stops*

But it all came down suddenly.
They had nothing left.
It all fell, like the trees they cut.

*Rhyming starts again*

---

*Story continues and rhyming persists till end of book*

Answer 1

It is definitely a literary device.

The only name I can find for it is a "deviation" See the paper "Internal and External Deviation in Poetry" by Samuel R. Levin, which says:

Thus, in a poem where a certain rhyme-scheme has been, or is being, established, the use of a non-rhyme would constitute an internal deviation. Such an occurrence, by frustrating our expectations, would call attention to itself and thus produce a stylistic effect.

To be more specific, that paper would classify this as an "internal deviation" because it differs from the rest of the poem.

Calling it a "break in rhyming," as you do in the OP, would be more specific and probably clearer.

Answer 2

If those three lines were spoken in a completely different voice or manner, as if spoken by someone other than the person performing the rest of the poem, it might be called a "voice over":

Voice-over is a production technique where a voice—that is not part of the narrative —is used in a radio, television production, filmmaking, theatre, or other presentations. The voice-over is read from a script and may be spoken by someone who appears elsewhere in the production or by a specialist voice talent.
— Voice-over - Wikipedia

There's not enough context here to say for sure, but in this case, the person telling the story might suddenly stop, pause, directly face the audience and speak those lines in a normal, non-poetic, non-performing voice. Then another pause. Then back to the story.

Or, a completely different person might step in, stop the action, say the lines, and then allow the poem to resume.