I fed the squirrel in the park The peanuts Uncle bought me, I held them in my fingertips The way my auntie taught me.
The gray squirrel sat and nibbled the nuts As the squirrels in the park all do. I sometimes wish that the little gray squirrel Would feed me peanuts,too.
For when he'd eaten all the nuts, Away the little squirrel flew. Do you think that was a very nice thing For a little squirrel to do?
A young speaker goes to the park, feeds a squirrel with peanuts, and then feels a bit cheated when it leaves. The details of the scene are small and affectionate: the peanuts came from an uncle, and the careful way of holding them, in fingertips, was taught by an aunt. The child clearly takes the whole thing seriously, as a proper exchange between two parties.
That is where the humor comes from. The speaker wishes the squirrel would feed them peanuts in return, and the final question, "Do you think that was a very nice thing / For a little squirrel to do?", is pure childish indignation. The title sets this up, since the squirrel never said thank you. The squirrel simply does what squirrels do, while the speaker expects manners.
The mood is light, gentle and playful. The simple rhymes and sing-song rhythm suit a child's voice, and the poem invites us to smile at the speaker's logic instead of correcting it.