About the theory of her parents being involved: most who have shared their theories mentioned an accidental death from abuse followed by a cover-up, and here is why I don't believe that:
It would not be easy to accidentally kill a strong, healthy, sporty six year old with a spanking or a fall after a shove. This is not a shaken baby, it is a strong little girl, and I am not saying spanking necessarily the correct way to discipline a child, but a six year old can take a spanking. Kids this age fall off bikes, wrestle around with each other, have sports injuries, if they die from a beating, it is a deliberate, extreme, forceful, excessive, intended injury beating. The idea of a child this size being spanked and falling and dying by hitting their head is far up there. Some kids ARE sadly beat this way (poor Nixmary comes to mind), but these are cases where there is a past history of abuse or suspicion of abuse, going to school with bruises, being dressed in long clothes to hide bruises, missing school, sports, etc. You would have siblings saying, "mom and dad paddle us". (Bianca) You would have teachers saying something. These things happen more to abused, neglected kids who are not enrolled in activities that the parents take part in, like baseball.
And non-abusive parents, in my opinion, would not go from zero to killing their child in one day.
Regarding the theory she died from abuse (sexual) by uncle: yes, sexual offenders will molest/rape and kill their victims sometimes, especially in stranger abductions, But while family member sexual abuse is very common, it is usually something that takes place over months or years, and these types of pedophiles, while sick and horrible, trend towards creating a "bond" and "secrecy" and (vomit) "special friendship" with their victims. I just can't see the uncle deciding to molest/rape and kill his niece and stage a kidnapping. If it was his first time trying something, and she resisted, it seems he would have denied it to the parents, said he was just checking on her or something, and moved on to a different victim. With no SO history or child predator history, a rape/murder is not a common first offense.
I am not saying it is not possible, but I do not think an accidental death by abuse, either physical or sexual, is the answer here. And even more strongly, I don't buy the idea of the dad helping the uncle cover it up. If a male relative of MINE hurt my kids, their dad would either kill him or call the cops, but not help him stage a kidnapping....JMO