Shortly after my son was born I started calling him "Buddy." I love it and he answers to it like a name now. My daughter is two and she calls him Buddy, which I think is the most adorable thing ever.
I'd like to do this with my daughter, but I'm not really a fan of things like "honey" or "sweetheart," though I do wind up calling her sweetheart pretty frequently.
Buddy is like friend, which is what I'm going for, but that's taken already. What else could I use?