The "D" in "DS", "DD", "DW", and "DH" can mean "darling" or "Disney". Both are commonly assumed/used on these boards (as indicated in the abbreviations thread).
In the first thread I read on the DIS boards, I was reading about someone taking herself and her DD somewhere... I specifically remember this because my first thought was "isn't this a family friendly message board?" I put one and one together and realized that DD must mean daughter somehow. Thank you for clarifying exactly how it means daughter! I was thinking Disney daughter too.
Although I guess the first D could be Disney here, the DD, DH ,DS, DW etc abbreviations are widely used outside the DIS on message boards of all types. So generally assume the D means "Dear" or "Darling".