Lines 12-15 Summary

Get out the microscope, because we’re going through this poem line-by-line.

I am a jack-o'-lantern
With terrible teeth

By line 12 the transformation of those "yellow balls" into full blown "jack-o'-lanterns" is complete. So we're in full swing of Halloween with all the festivities and that emblematic jack-o-lantern that sums it all up.
And just like in line 5, when the speaker says, "I am called pumpkins," he says here in a similar way that "I am a jack-o'-lantern." Now he's singular instead of plural, though.
In this case, the speaker is repeating that "I am" clause, which really helps to convince us that the pumpkins are telling this story. It's almost as if the character of some Halloween ghost story popped right out of the pages and is coming to life right before our eyes in this poem.
We also have some alliteration here in "terrible teeth," which accents the appearance of those frightening teeth while also using the same sort of fairy tale language we saw earlier. Again the speaker is keeping this poem kid-friendly and reminiscent of all the stories children might hear. Check out "Sound Check" for more on the sonic effects at work here.

And the children know
I am fooling.

By the very end, the kids know that the jack-o'-lantern with "terrible teeth" isn't something to really be frightened of because it's just "fooling."
So, the kids and the pumpkins seem to have an understanding around Halloween that, sure, things may look scary, but it's all in good fun. There's no real danger here because the children "know" what's really going on.
Notice that "I am" clause popping up again in line 15. So even though we don't have any fancy meters or rhymes going on, Sandburg is still using some fancy footwork to keep it all sounding poetic.
It's also fitting that Sandburg would end this poem with a final declaration from the pumpkin-speaker, since that jack-o'-lantern is the star of the show.
So, by ending it all with our attention focused on that pumpkin voice, Sandburg is able to make a rather short poem just as memorable as a long one. All it takes are a few strategic reminders and some repeated clauses that tell us this poem is coming directly from the source: pumpkins.
And since it's not every day we get to hear pumpkins speak, it's a safe bet that "Theme in Yellow" is pretty special—in terms of its speaker, voice, and overall mood.