I remember sitting in the back seat of Seth and Catherine's rental car one Passover, Henry in the middle and Andrew on the other side. Andrew called Henry "Buddy" and Henry, in his little not-quite-one-year-old voice repeated the word - "BUD-dy." It was so cute. Then, whenever Hen would see a picture of Andrew, he would point and say excitedly, "Buddy!"
I don't know when I started calling Maddox "Buddy," but it seems to have stuck. Sometimes I feel bad, because it seems sort of generic (on the other hand, he may be perfectly glad to be called Buddy rather than Moshe PuPu, as we sometimes call him now), but I can't help myself, it just comes naturally to my lips.
Chava has never really jumped to call Maddox Maddox. I don't even think I can put letters to what she does with the pronunciation of his name (although I should try, because it might be just as fun as telling you that she calls the rabbit at M's school the "yabbit"). She's got the "Ma" part down, it's really the second syllable she has a hard time with. But she's solved that problem. She just calls him "Buddy."