Nicknames for the ones I love is just something I do. Don't know why, but terms of endearment just happen. Shortly after Bill and I started dating seriously (and I think after we were engaged), Pookie Bear became a name that we share for each other. Pookie Bear evolved into Pookie and Pook. In addition to this, I call Bill "Billy" often.
After Logan was born he had an array of nicknames: Grumpy-grump, Mr. Fussy Pants to name a few, but Sweetie Pea stuck and I call him this more than he'd probably like to admit when he realizes what I dork I am. Shortly after Carter was born I thought about nicknames for him, but not very hard. Such is the life of the second born child. Somehow, Baby Bean came about and automatically became what I lovingly call him.
So, Sweetie Pea and Baby Bean are their nicknames - don't ask me why they are named after legumes.
And don't ask me what I call them all when I'm in a bad mood, either.