おむすびころりん reading help

Reading this 1年生 book and there’s this sentence that starts with と that confuses me. It looks like it’s a quotation particle, but there’s a full sentence before it? What is it quoting? Clearing the character didn’t hear a song that included the beginning of that sentence, so do I just assume it’s the onomatopoeia? It’s the third sentence…

So, here, the whole

むすび こらりん
すっとん とん。

is being quoted. Taken all together, those first four lines say something like:

Upon peeking in, a song that goes:

”musubi kororin
”sutton ton”

could be heard through the crack.

It probably is onomatopoeia describing how the song sounds, yeah.

If it helps, “という” is also written as “と言う.” Also, Japanese will often break quotations and their tags into multiple paragraphs where English will often keep them in a single one. I think the quote itself was probably split into two lines here in order to better convey the rhythm of the song.