I read about two other seemingly similar cases today.
Jhessye Shockley, Arizona, 5 years old, amber alert issued in 2011 after her mother reported her missing from the home. Mother later convicted of 1st degree murder even though no body was ever found.
Sylar Newton, 2 years old, disappeared in 2010 on a camping trip. Skeletal remains found 2 weeks later by a volunteer searcher (and distant relative of Sylar), in a spot in the river that had already been searched by search and rescue several times. LE said the body had been intentionally hidden, and that's why they didn't find it - they weren't looking for a body that had been hidden, but a toddler that had wandered off.
The mother later confessed that the child had gotten into some drugs she had brought - klonopin. She said he was groggy and falling down when they last saw him alive. She had also brought cannabis and methodone on the trip.
Cause of death could not be determined from the remains.
The mother and grandmother were convicted of child abuse, for failing to call 911. The mother got 17 years in prison, whereas the grandmother just got lifetime probation.
http://dcourier.com/main.asp?SectionID=1&subsectionID=1087&articleID=107305
Collection of archived articles about Sylar Newton's case:
http://www.justice4caylee.org/t7623-sylar-newton-2-yo-2010-camp-verde-s-of-flagstaff-az