This is just my opinion, but if I saw a childs backpack sitting on a curb alone, I wouldn't think anything about it. The first thing that would come to my mind is that a child sat it down and went to do something else.
I am guessing that the backpack was placed after she was murdered, or at least disposed of, as a taunt. Based on my knowledge of these sort of crimes, I am guessing she was murdered within the first 48 hours of abduction, then there was a bit of time (a few days) to contemplate disposal, next steps, etc. No matter how long someone plans such a thing, there are many variables that cannot be predicted in advance (LE and community response, time until body/clues are discovered, possible witness, etc). These considerations probably take a few days to arise and sort out, and undoubtedly determine when/how to proceed.
Really is a sick world we live in. To those of you with children: You can never ever be too careful. Often, the first time you allow your child to do something a bit outside your comfort zone could be the last. I have read so many stories that end with a parent saying, "He/she had been begging me to do this (walk to school or do something else alone, etc.) for months and we planned it and practiced it, so we allowed him/her to do it. He/she was only out of sight for 1 minute/1 block." Well, sadly enough, there are people out there looking, seeking, hunting, for such opportunities. It is just not worth the risks. And if you think that your neighborhood is "safe", well, criminals know which communities have their guards down due to a false sense of security. Sad but true.