I think that's a Southern thing.
"Pumkin" (the P gets lost in Georgia's drawl) was what my Papa called all the girls. Along with "Lady bug" and a couple of others.
When my oldest son was about 3 we had a hilarious conversation with him one time called "What does ____ say?"
What does GranPapa say? "hey Dude"
What does GranMama say? "hey pumkin" (yeah that one got passed down!)
What does Daddy say? "hey kiddo"
What does Mommy say? "hey sweetie"
He knew exactly what everyone's nickname for him was. It was so funny!

(I haven't thought about that in years - thanks for bringing back the memory!)