Okay, this one is probably the most obvious symbol in the book, but we have to call it out. Take a look at stanzas 14 and 15:
You'll come down from the lurch
with an unpleasant bump.
And the chances are, then,
that you'll be in a slump.
And when you're in a slump,
you're not in for much fun.
is not easily done. (14, 15)
Now look at the illustrations. Those blobby things represent your child's current slumpy state. And they're slumping over a cliff, so let's hope they get righted sooner rather than later.