Having no way of knowing how accurate he was, I remember that on Fox News they were discussing another recent missing child, and someone asked this very question. The forensic pathologist said that it begins immediately. Sorry I can't remember the doctor's name.
Well, it surprised me...since the poor little girl’s body was transported only a couple of hours after her demise, in the middle of the night...in November.
Decomp would begin immediately. However, keep in mind that she was in a cave, where it is cool to begin with.. and for most of the last week we were having a cold snap. Not so cold as to prevent decomp.. but would slow it down somewhat.