Pity (for E, aged three)

I picked a beetle up and let it go,
And that was pity;
But not the pity that you could not go
Today, as you’d been promised, to the Zoo
Because you were too sick.
So ‘What a pity’ were the words I spoke,
And then you asked your question: ‘What is pity?’

I’ve searched and searched, but can’t find out the trick
To tell you what it is. My glib words choke
Attempting to spell out the different sense
Of rescuing the beetle, and why you
Did not go with us to the promised Zoo.
What is the difference? How can I say
That pity is a debt I cannot pay?